The 35i normally has 300 lb-ft of torque, whereas the 35is normally has 330 lb-ft of torque, but for short bursts maximum torque with overboost function increases to 369 lb-ft.
The 35is includes the M Sport Package which is a option on the 35i.
The 35is has different front and rear bumper fascias.
The 35is has computer programing differences that are designed to make the steering, ride and shifting somewhat more sportier.
The 35is has a number of interior trim differences, which in some cases are included in the 35i M Sport Package (e.g., steering wheel) or may be unique to the 35i (e.g., door threshold and floor mats)
Couple of exterior color options are different. Example: Crimson Red is not available on the 35is, and Melbourne Red Metallic is available on the 35is, but available on 35i only if you get the M Sport Package.
Here's a couple of links:
2011 35i and 35is specs, packages and options in PDF format
Z4 Series Overview
You may also download a free brochure