Let one of the few guys here with a 318i set the score straight

ekoUK, things are a bit different here in the US with the 318.
The 318i does have rear disc brakes. The 84-85 M10 based 318 has the rear drum brakes.
The differences were mostly minor like said about - trim, seats, wheels, etc.
You can go on realoem.com and compare the part numbers:
Same front sway bar
Same front springs
Same front struts (51mm)
Same control arm and control arm bushings
Same steering rack
Same options for rear sway bar - 12mm stock, 14.5mm "ONLY APPLIES TO M TECHNIC"
Same options for LSD
Same brakes
Same lack of full OBC and cruise control options
Seems to me that the 318i and 318is should be pretty evenly matched performance wise. 318i is about 50lbs heavier but we all know sedans are stiffer
