Pretty much agree with the forum consensus. 335d wins on low end torque and mileage; 135i is more tossable due to less weight; 335i is an all-rounder. And you get get gas anywhere, diesel is sometimes a pain.
But I'll comment that you don't need to get too hung up on HP vs. Tq. They are literally the same number with a different measurement system (HP = Tq * RPM).
(Purist note: simplifying here.)
It's really the shape of of the power curve that matters. At what revs does the engine deliver the power (expressed as HP)?
Usually people talking about Tq are talking about power at the LOW revs. AKA 0-60 drag racing power. Low-end torque is fun and easy to drive at low speed. Step on the pedal, even at low revs, and the car moves. You can't bog the car down. Remember HP = Tq * RPM. With enough Tq, even low revs give you good power.
Here's the catch. Big Tq requires big engines. Big engines explode if rotated very fast, so the rev limit is low. Big engines also lose Tq quickly at high revs. This means an engine tuned for low end Tq often have no power at the high end. This is why a tow truck can't go 200 miles an hour.
Instead, imagine a smaller engine that rotates much faster. Again, HP = Tq * RPM, so if there are enough revs we can get power that way. Thus, people talking about high HP are usually talking about power at high revs.
Cost being equal, high revving engines have higher max HP than big engines... but that power is harder to harness. Keeping an engine at high revs all the time takes skill and annoys your neighbors. Thus casual drivers like low-end Tq, to drive around town. Racecars drivers want more revs and more peak HP.
Turning to BMW specifically:
The 335d engine (M57) is a "big" engine. HP/Tq: 280/425. It has much more low-end torque, but can't deliver power above about 4500 RPM.
The 135i/335i engine (N54/N55) is balanced between big and high revs. HP/Tq: 300/300. It is well-rounded with good torque and good max revs (about 6000 RPM).
The diesel engine is a BLAST at lower speeds and city driving. The gasoline engine is a better track engine.
Oh, and the 335d only comes in automatic. That was the dealbreaker for me, I need my three pedals!
BMW CCA Member