I chose the M4, which may surprise some people as I have an M2 being delivered in about a month (assuming the rear differential issues BMW are having don't affect it).
The M4 is far better value than the M2 when you consider the discounts available compared to the zero discounts available for the M2. The ability to spec the M4 a lot more than the M2, and therefore personalise it, is also a factor. I also think the M3/M4 is better looking, and is also the sweet spot between something fast and exotic, but not too exotic that it changes how you drive it, where you park it etc. That is something that affects my enjoyment of my R8.
I have neither the driving ability nor a private track at my disposal in which to exploit either of these cars to the fullest, in which case the subtle differences in handling would be lost on me.
The reason I am taking the M2 is because the expected residuals are such that I probably won't lose too much money (if any), and it's a way of experiencing another car for a while. I plan to replace the M2 with an M4, but if I could only have one of those it would be the M4.