This is my opinion, but there is a bit of fact to it, I believe.
While Alternity is the newer line, and thus features newer model cars, I would easily recommend Alternators/Binaltech over it. Bigger toys and not as fiddly. I also think the fit was a bit more precise.
There are some Alternators that are notorious for their difficult and frustrating transformations: the most notorious being the Grimlock/Wheeljack mold (Mustang).
I hope you've got enough info to get you started, and good luck.