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.
2015 M4 Mineral Grey with SO, 2015 BMW i8, 2014 Hyundai Santa Fe, 2015 Fiat 500e
2013 Tesla Model P85 (sold), 2010 RR Phantom, 2014 S550, 2012 Porsche Panamera S