The P-bass GAS is inevitable, it's just a matter of time.
In the case of the P-bass, if you have the option to go American then you should definitely do that. You can make an MIM or MIJ sound like an MIA, but if you are one who appreciates excellent build quality, fit and finish, then the MIA is totally worth it. The finish is better, the bridge is nicer, the pickup on the MIA is a custom shop, the body is made out of fewer chunks of wood so it resonates better (and looks much better through the finish), the fingerboard has rolled edges, there are some small details like the string spreader and saddles being cast rather than stamped, the neck has graphite reinforcement for added durability (even Billy Sheehan couldn't snap a P-bass neck!), the fret dressing is done better, and probably a few more details I'm forgetting.
If you are simply GASing for the tone and nothing more, then go for a used MIM and have a blast! (maybe just upgrade the pickup if you desire), however, if you want to own the icon then go for the American, no doubt.
Also, don't forget that tortoise shell pickguards improve your tone by 180%