[Deschodt]

C4 cab - hmmm, I'd prefer a cayman over a 4wd convertible 996. Also I think the 996 interior aged poorly.
But, the 996 engine can be IMS'ed, the Cayman's in 2007 form cannot. Also the 996 will have more power... I'd still get the cayman between the 2, newer, more grip and handling than power, you cannot go wrong with that platform. It's the best handling Porsche I owned until the 991 came along and stole some its "turn in" magic. Had the 996 been a gen2 RWD coupe, it might have been closer for me.
Honestly I'd stretch for a 2009+ Cayman and not worry about the engine, you're taking a 15K gamble on a 20K car (unless you redo the IMS and pick the 996).