This is from a guy that used Postality, THEN the original paid version:
It's all in the scripts. When i started assembling my game, i did it in postality and added a load of scripts - when i started using the official paid version, i direcly ported the game, but decided to redo all the changes on top of the oficial scripts, so i could keeping the translated comments in them. Result? Bugs and more bugs!
It doesn't matter if you're Postality or official: if there's a bug on the scripts, it will crash. Scripts made to be used with postality show incompatibilities with the official, usually on combinations hard to be detected (like, mine crashed only after a "continues even if defeated" autobattle), etc. The official version with NO extra scripts are completely bug free... if you think that a standard game made with THAT is interesting. Otherwise, postality will give you the chance to use scripts made FOR postality.
Add battle, message and menu systems and you're bond to run into bugs, no matter your version.
Official RMXP gives you a chance to SELL the game, and that's all.