I think it really just boils down to what you want to do with your car. If you are going for a purely cosmetic standpoint, then I would do the kerscher bumper. It will be much cheaper in the long run of the project.
If you are going for performance and want to reduce understeer, than widening the front of your car and running a square set up will greatly improve your car.
Do note, if you get the 1M bumper, and fenders, you will also need headlight brackets, new windshield washer reservoir, all bumper/fender trim pieces and you will have to custom modify your 135i sideskirts to line up with the 1M fenders.
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|