The two are incredibly similar in external appearance. There are people that don't like the 1 series look, which i get, but its pretty rare to find the 1M attractive and the 135 ugly because they are so similar. Personally, I much prefer it to the 335. It has the same turbocharged engine that puts out identical horsepower but in a smaller and lighter frame giving it greater acceleration and a more torquey feel. The better power to weight ratio has spoiled me and now whenever I'm in a 3 series I find it sluggish.
I also like it because it seems like its more of a cult car. You can walk outside, throw a rock, and hit a three series. This isn't a BAD thing i guess, I mean they're popular and dependable cars. But the 1 series is much less widespread. Everyone I meet who has one seems to be a pretty big BMW enthusiast, not someone who just wants a car with a BMW badge.
Smaller, faster, sexier, more unique. Thats why i went with the 1.