So, I have had my GTD for 4 full days now and my g/f has a 120d M-Sport.
Here are my initial thoughts in comparison:
Performance:
Difficult to tell, my GTD hasn't cracked 250 miles yet and the BMW has ~5,000 miles. The power delivery is certainly different, the BMW more old school diesel, the GTD has a smoother less peaky delivery. I suspect there would be little to choose between them, there is less than half a second in it in the official figures for 0-60 (for what it is worth) and I reckon from a rolling start from 20mph to say 80 it would be close. I think we'll call it equal.
Ride and Handling
Here, there is a difference, the ride in the Golf is simply better (I have the standard dampers) than the BMW which is fidgetty at low speeds. The handling is more confidence inspiring in the Golf but I think the majority of that is down to the Bridgestone tyres on the BMW, this expresses itself in the BMW as, even in Sport, as a weird numb lightness in the steering on initial turn-in, the car doesn't understeer as such but its almost like a video game in that you just have to rely on the fact the car is going to turn... not good, but as I said I'm pretty sure its the tyres. The Golf has a chuckable, unstickable sort of feel to it, although you are always aware of the slightly nose heavy attitude compared to the BMW. The Golf wins by a small but significant margin.
Economy
Difficult to tell at the moment. The BMW averages >48 mpg with mid-fifties achievable on a motorway run averaging 75mph. I can't yet comment on the Golf, as I haven't been on any longer journeys in it. The nearest I have had so far was a 28 miles trip of mostly motorway/dual carriageway with cruise set at 75mph and I saw 46.6 for the trip which included some heavy, stop-start traffic for 10 minutes at the beginning, so not too bad.
Interior
Both cars have very different concepts. Firstly, the BMW, has the feeling of a cockpit, the seats are excellent (significantly ahead of the Golf, imo) and you can (and do if you're me) sit very low... but it feels a little dark and cramped in there; however, it is well made if a little scarce on the equipment side. The Golf has a lovely airy cabin the downside of this, for me at least, is that you sit too high. The seats are acceptable and it is well equipped and appears beautifully put together. So, the Golf would be difficult to beat, if I could have the BMW seats and driving position. There is more space in the Golf, in general but I think, I'll have to call it a draw.
Just a quick note, the BMW has the Harman-Kardon upgrade and I have Dynaudio in the Golf. The Dynaudio is the better sounding system to me (and the g/f) although the BMW system is still pretty good.
Equipment
Easy, the Golf wins hands down. Connectivity to the stereo, adaptive cruise control and those lights, both inside and out. etc. etc.
That's all for now but I may get the opportunity to have a run in them both soon with the Dynolicious app (when the Golf is run-in) to compare the cars objectively from a performance perspective.