This is a topic where I have first hand information
. I have a 16R, which is a great car for what it is and I certainly consider it the best car under $50k in Canada.
Fast, fun to drive but it has its drawbacks. The engine spins at a very high rpm when cruising because of the weirdly short gearing, the car also suffers from significant turbo lag and is basically dead below 2.5k rpm. And the hatch is useless if there are people in the back seat.
But the interior quality is great, more Audi than VW and the car has a great sleeper charm to it.
The 340i I have has the full leather dash so interior feels a notch or two above the R. Also I have the digital display so it makes it feel more flashy.
The biggest difference is the engine. The B58 with the MPPSK is significantly more powerful (355 vs 292) with much more torque as well (369 vs 280) and this difference is very noticeable when on the road.
Golf R also sounds lame if you disable the fake sound it emits via a dash mounted speaker ( soundaktor). The 340i sounds like a beast.
This comparison is honestly one sided because golf R is a significantly cheaper car and in a different class as compared to the 340/440.
Pictures of both my rides .