I got my 07 335i about a week ago, and I'm in love with it. Haven't driven a 135i, but I can strongly recommend a 335i. It's a great balance of comfort, handling, and power, all while being a bit of a sleeper.
335i is OK, 335d is better, but all BMWs are overrated anyway.
Eh, the diesel life isn't really for me. I haven't really found anything about this car, or any other BMW I've driven or been in, to be overrated.
The N54 is really sweet, you'll love it. I don't recommending modding it, but if you really must, there are lots of ways to get easy power out of it on stock turbos and internals. Downpipes/intercooler/ECU tune is an easy extra 100hp, but I'm really not a fan of modding power without modding suspension/handling.
There really isn't anything else there that has the balance of power, handling, comfort, electronics/entertainment out there for the same price. It's a really well balance of performance and minimal luxury!
I've started thinking...
List what you guys think are wolves in sheep's clothing.
Example: 135i N54 and 335i. Also the new M235i with the N55 has a forged bottom end.
I really like the N54, since I daily drive one, but I don't think these cars are considered sleepers, unless you compare them to exotics.
That said, no one will expect a N54 335i to have 650hp, and handling/track suspension of a M3, if you decide to go that route and mod it with a full suspension overhaul of M3 parts, coilovers, and upgraded turbos. That's a lot of money.
You'll always have people who have minimal knowledge of cars look at cars solely based on what brand it is
Sleepers are probably more like... hmm, I'd say the new Tesla P85D... so sleeper that you won't even hear it coming