Do you plan on modding the car?
I ask because most of the stuff that the 135IS offers over the 135I are things that people usually mod first. The 135IS comes with bmw performance wheels, bmw performance exhaust and the bmw performance power kit. All are great upgrades from the factory but there are lots of aftermarket offerings that will match them in quality and often in price. Also modding yourself, you get to pick the parts you like and make your car suit your own personal taste.
So if you just want the car out of the box with no intention to mod and can afford the price increase, I would get the 135IS. If you do planning on moderately modifying the car, I would get the 135I, a Cobb AP, Apex wheels, non runflats and a berk exhaust and then save for some suspension mods. The above 135I will smoke the stock 135IS.
The Porsche is a whole different beast. I really like the new boxsters and they really have strayed away from the 'chick car/poor man carrera' boxsters of the past. The 135I may be quicker in a straight line and on paper, but the boxster will likely smoke the BMW on a track and be just as, if not more, fun to drive.
The bmw has more luggage space and can fit two more people in its foldable rear seats if that matters.
MY Build Log
08 AW 135i | Cobb | ETS FMIC | ER CP w/ Tial Bov | AR Catted Downpipes | Berk Mids | Meisterschaft GT SUS 1M |M3 Control Arms | M3 Sways | M3 Rear Subframe Bushings | Wavetrac LSD | AST 4200 DA Coilovers | HRE R43 | Hankook R-S3 |Full BMW Performance Interior | Revozport 1M Raze Aero Package in Satin Carbon |1M Body Conversion|