Underpowered or not really depends on the driver. For every 318i driver you see saying it's underpowered, you'll find another saying it's not. If you do drive aggresively, then yes, you might find power a little lacking. After all, it's a 1.8 engine with 1.5 tonnes to pull. To compare it with the Perdana is comparing a 4 pot engine with a 6 potter. Yes, I think the Perdana is lighter (I may be wrong). Yes, the Perdana has 2 more pots. But mind you, BMW does a lot more with an engine than a Proton. So I wouldn't say the Perdana has won it.
If you're thinking driving comfort and safety, then a 318i is enough for you.
If you're a Jekyll/Hyde person like me who, once a while, wants to tear down the highway screaming 'Power! POWER!', then go for a 325i or 328i. It sure beats trying to modify a 4 pot later. I'm very happy with my E36 325i.
When deciding which one you want to get, I'm afraid it isn't us who should tell you which is better as both have their well documented advantages (318i road tax not that sakit mah). It's you asking yourself what are you really looking for.