I don't think he is making a "mistake", 325 might seem like a "mistake" to some of the 330 owners, but when some buys 325 over 330, they probably have few reasons why. They know themselves better than anyone else, and Im sure they made the decision based on what really suits their needs.
As far as which engine lasting longer, both are the same engines. And if anything happens(both equally abused), 330i engine probably will go away faster due to more stress to the engine because of higher power output which is obtained simply by more tuning. I doubt this will happen though. I don't think BMW engineers were stupid enough to design whatever engine in the 3 series(which contributes 60% of total BMW sales) to be unreliable and just die off because of some abuse. I don't think you really have to worry about that at all when choosing 325 or 330.
Both are great cars, and lets not put down 325 or 330. If we do, we are basically putting down our own cars, as both cars are virtually the same.
(though, few members in E90POST have traded their 325 in for a 330) 