I voted 275 / 285. As previously stated, there is no reason to run a square set when you can't rotate them anyway. I'm always for running the beefiest tires you can. If the PSS are less expensive then it's a not-brainer to get them (great tires). You may need some camber to run the 275 in the front. If you have KW coil-overs you will also need a 5mm spacer in the front to clear the helper spring.
'13 SSII M3 Coupe - KW Clubsport 3-way (Eibach Front Spring Conversion), Akrapovic Evolution, ESS Tune, StopTech Trophy, IND Euro Front Bumper and Gloss Black Mirror Covers, TMS Studs/Lugs, BMW Performance Carbon Fiber Spoiler/Interior Trim/Center Console, BMW Edition Grills/Gills, BMWPedals Extended DCT Paddles, Mode Carbon GT4 Splitter, Advan TC3 (18x9.5/10.5), BBS E05 (18x10/11), G-Force Rivals (275/35/18, 295/35/18)