Of course power wise, 528i will anytime outperform a 523i. It goes w/o saying. But if $$$ isn't a issue & if you would someday think of doing an overhaul (for engine restoration sake), there are a couple of "hantus" than can be done to a modest 523i to deserve the calling "a wolf in a sheep's clothing". In the end, the idea is to continue paying the roadtax of a 2.5cc with extra OOOMPH!!!:top:
Then again, you get quite a number or goodies in a 528i that comes as standard accesories. As for me, engine capacity isn't much of an issue. As long as it's a minimum of 2.5cc straight 6, it's fine for me.
If you want my advise on buying a used car, take a read at post #56:
http://bmwclubmalaysia.com/forums/showthread.php?t=27143&page=6
-my 2cents worth