European 325i/330i post circa 2007 both use the N53B30 - engine is 3.0 in both cases, 330i has a different inlet manifold & ecu map. To get 325i to 330i you either go aftermarket remap (260), three stage inlet manifold & aftermarket (270) or inlet & OEM 330i map (270), or figures along those lines. The 330i's three stage inlet manifold gives better torque.
Earlier European 325i has the N52B25, 2.5 litre engine, not upgradeable to the higher power as per the N52B30 in the 330i. The US 325i has the N52B30 just without the 330i manifold, hence they upgrade it. To confuse matters, Europe got the N52B30 in the 125i, so that can go from 215 to 265bhp with an inlet/map change.
The "unicorn" cars in the UK are therefore the N53B30 325i and the N52B30 125i where a manifold swop & map will give you an extra 50bhp
In terms of reliability, the N52 engined cars are arguably better due to their simpler injector design (the N53 is more like an N54 without a turbo, so you have HP/LP fuel pumps, NOX sensors & Piezo injectors to worry about), but realistically speaking, both should see at least 250k miles. The Americans have plenty of N52's on more than that. Also, from a DIY perspective, the US gets the N52 and they love a good DIY guide, they dont get the N53, which makes things a little harder. Ive been having to use N54 guides as the basis for my engine work on the 325i.