Of course you're going to get biased answers here, but I agree that the M3 is more reliable than a 135i, though running costs will undoubtedly be higher. I'd check with your insurance and such and then try to test drive an M3; just make sure to do a proper test drive, i.e. stay ABOVE 4K and in lower gears for extended periods, because if you just cruise around it won't feel as special. Then make your decision from there -- though I'll give you fair warning. If you test drive an M3, you WILL want to buy it.
Having driven an M3 and a 1M (never a 135i, sorry), I came to appreciate the 1M as a car that's far easier to accelerate quickly at civilized RPMs in casual driving scenarios and thus can make sense for someone who just needs a fun DD. But when you want to really go out and DRIVE, there's no comparison to the M3. Just make sure you'll be visiting a track or at least some twisty back roads frequently just for the pure joy of driving, because if you do nothing but DD your M3 along city streets and freeways, you'll probably see it as a waste because your current car is better for that. But honestly, even twisty back roads don't compare to the track in terms of helping you appreciate the M3's existence in this world.
Also, if you buy the M3, since it's an 08, try to get your software updated at the dealer. A friend of mine still had the original factory software and got the latest stuff, said it definitely improved the engine feel, which seems to be others' impressions with the very latest round of updates.