I've had both
They're both good cars
Depends what you want from the car, for most people the 325i is the better choice - you can drive them quite lazy, pull well etc etc
If you dont mind driving like in a race everywhere and revving the tits off a car, keeping it in the power band then the 318iS does not have a huge difference in speed to the 325i as it had lower gearing and weighs less (75kg before other equipment according to the manual - not sure how much actually in real life, possibly less)
You'll drink as much gas as the 325i though driving like this
Of course straight line speed isnt everything
The 318iS is def. better through the corners than the 325i when you're pushing on
In my experience a 325i wants to push wide on turn in, and then from the apex will oversteer if you wish
A 318iS is much more balanced/neutral on turn in and exit
Of course for regular driving this isnt a concern, and I'm talking stock suspension (which is mtech on the 318iS)
I'm sure I've posted slalom speeds for 325i vs 318iS somwhere here before that prove the 318iS is quicker though the bends, do a search
For me the 318iS is the more fun car to drive, you have to work harder to keep the speed up, but then its more rewarding, and you can still scare yourself silly down a decent back road
And you can get decent MPG out of it if driven sensibly
If you're more a lazy or straightline speed kinda guy then go for the 325i
I must admit, sometimes I miss the sound and torque of the 325i and I'm occasionaly tempted back to the dark side
