Personally I think go for what you liked to drive the most... Then look at how you use it... then look at the financial side... then pick best compromise. Personally I hated the A45 and CLA45 - one drive of the A45 was enough to rule out the Mercs.
The RS3 was really, really good but this is where the usage comes into it - I wanted a fun weekender, not a daily driver, so the M2 in manual easily won over the RS3. Had it been a daily driver I'd probably have gone the other way...
Depends also if you like traffic light grands prix, if so the AWD works better for launches. As I track the M2 it was a no brainer - it is serious fun on track.
I also might add I don't get the whole "pick the right colour and options for resale" thing. Buy what YOU want and enjoy the car.
My $0.02.