Thanks for the reply. I have an M-Sport car now and there are some things I like about the package, but it may not make sense given my needs and I care more about getting the car I want than resale.
Unpopular as it may be with others, I prefer the chrome window trim over the shadowline trim, especially on the 1 Series. The windows on the 1 are already relatively small and the beltline is high, so the dark trim just accentuates those proportions to my eyes. Also, the 128i is going to be my last "old school" BMW (NA straight six, manual transmission, etc.) and the chrome trim reminds me of the E30 days. I'm on the fence about the M-Sport body kit, but I kind of like the simpler standard body for the 128i -- again, it looks more like a traditional BMW to me. As for the wheels, I'd be dumping them for 18s, so no gain there for me.
What matters most, of course, is the way it drives, hence my original question. If the M-Sport package + Performance Suspension offers any performance/handling advantages over Base + Performance Suspension, it's M-Sport all the way.