I picked up a 35i + ESS stg 2. I, personally, didn't think the 35is was worth the extra cash even if it is a tad bit stronger. I put a lot of that upgrade money into KW, upgraded wheels, intercooler, DP, etc and still pocketed some change. My butt dyno says 35hp increase between the two won't be missed.
And I guess I'm lucky with having STBMW near me so I don't have to worry about BMW a-holes flagging my car....but if I had to, i don't think I'll have much problem putting the car back to stock for warranty purposes. I almost never go to the dealer anyways.
2013 Tesla Model S, 2011 Land Rover Autobiography, 2010 RR Phantom, 2012 S550
2010 AW E92 M3 (SOLD) 2009 Z4 Sdrive35i (SOLD)