Ptolemy definitely drew on Posidonious rather than Cicero, but he used the arguments of Posidonius
against the criticisms offered by Cicero (who was a student of Posidonius, so knew where the arguments could be challenged). I'm including a lengthy extract from one of my articles because it is directly relevant to this. It was written for issue 20 of the TA in 1999, which never got published due to my illness (so I had to dig it out from the computer vaults

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The historian and philosopher
Posidonius (c.135-51 BC) was one of the early pioneers who had studied Greek philosophy in Athens and then later established disseminating schools in Rhodes and Rome. From these centres he transmitted the main principles of Stoicism to leading Roman students who in turn went on to become prominent orators and effective communicators of the principles inherent in astrology as it arose in the Roman world.
Posidonius was greatly admired as an influential academic. He dedicated much of his life to research and through his travels and observations he was able to prove the connection between the tides and the phases of the Moon, and demonstrate the circumference of the Earth. He was a firm believer in cosmic sympathy, approved of the philosophy of the Stoics, and advocated the use of astrology. But he also argued that theories need to be backed with evidence, so he spent much of his time collecting case histories to prove the viability of divination, and to strengthen his argument that the future is organised, not merely subject to chance events, and therefore able to be predicted. Unfortunately his own works are lost but his teachings have been conveyed through the works of his distinguished pupils and it is generally acknowledged that the success of Stoicism in Rome was centered upon his influence. Similarly, his professional standing lent credence to the rational pursuit of knowledge derived from the stars, allowing astrology to overcome the negative connotations of critics who wrote in the style of Ennius. Professor Franz Cumont, ? who considered Posidonius to be the true inspiration behind Manilius?s Astronomica and referred to his ?intellectual sovereignty? as ?the great intermediary and mediator not only between Romans and Hellenes, but also between East and West? ? saw him as a master among masters and claimed:
?? above all it was due to him that astrology entered into a coherent explanation of the world, acceptable to the most enlightened intellects, and that it was solidly based on a general theory of nature, from which it was to remain henceforth inseparable.?
Although the impression that Posidonius left upon astrology is enormous, it would be wrong to think of him as the originator of new ideas. As a key intellectual figure and a persuasive, forceful communicator he, like Zeno in Greece, brought attention to philosophies that, as Cumont puts it, ?must certainly have shown themselves long before him and abundantly around him?; much of Stoicism being rooted in Mesopotamian theology and various ancient oriental cults. But it is fair to look upon Posidonius as a philosophical champion who was responsible for a key movement of ideas at a particularly sensitive time, allowing those ideas to adapt to and cause the adaptation of related studies. Thus it is said that when Neo-Pythagoreanism emerged in Rome, ?it professed to connect itself to the old Pythagorean mysticism, [but] its doctrine owes more to the theories developed by Posidonius?.
One of Posidonius?s most illustrious students was the political orator
Cicero (106-43 BC), who attended his school at Rhodes in 78 BC. The strong impression that Posidonius left upon the young Cicero is evident in the great respect that the latter shows for his former teacher in his writings, even though he later came to differ with many of his views.
Upon completion of his studies, Cicero returned to Rome and embarked upon a colourful public career, eventually to be murdered by the soldiers of Antony in 43 BC. Towards the end of his life, when his political career was waning, he took to writing about philosophy, ethics and those subjects he had studied as a young man. Within these works he appears to have taken an ambivalent attitude towards Stoicism. Generally, he demonstrates agreement with its doctrines, (conceding in book three of De Natura Deorum that of the main philosophies the Stoic argument is more likely to be right) but in the work which immediately followed this, De Divinatione, Cicero set out a damning attack on the Stoic belief in Providence and put forward all the contemporary arguments against the use of prophecy. Here, for example, he speaks of the futility of prediction ? if the future can be foretold it is obviously preordained and therefore unalterable ?
?What is this premonition sent by the gods, this so-called warning of disaster? What do the immortal gods have in mind when they give us clues that we cannot understand without interpreters and against which we are defenceless anyway? No decent human being would do that: predict to a friend an imminent disaster from which there is no way of escape.... If the gods really wanted us to know the future, they should have stated it clearly, if not, they should not even hint at it.?
When Ptolemy wrote his defence against criticisms such as these, a century and a half later, he drew upon the earlier arguments of Posidonius for his response:
? we should consider that even with events that will necessarily take place, their unexpectedness is apt to cause excessive panic.... while foreknowledge accustoms and calms the soul and prepares it to greet with steadiness whatever comes. Also, we should not believe that events attend mankind as the result of the heavenly cause as if they had been ordained by some irrevocable command and destined to take place without the possibility of any other cause interfering. Rather, the movement of the heavenly bodies is performed in accordance with divine, unchangeable destiny, while the change of earthly things is subject to a natural and mutable fate, and in drawing its first causes from above it is governed by chance and natural sequence.
Because of the views set out in De Divination, Cicero has become known as one of the last great opponents of astrology in the classical world. Yet while he may have attacked their philosophy, Cicero clearly moved comfortably in social circles attended by astrologers and enjoyed their personal companionship...
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I use part of that in my course work, to make the point that even Ptolemy - although his astrological philosophy appears to be completely predeterminalistic - acknowledges the possibility that another ?cause? may interfere. So the possibility of chance or free will preventing celestial predetermination from fully expressing itself is never denied from a philosophical basis, even within the strictures of Stoicism.