I didn't count but atm it's probably a tie between 13" and 15"
Well battery life is rather nice to have for me. It's definitely not essential for me because it will be on my desk 80-90% of the time. ;-) The Late 2013 15" models also have quite a nice battery life with Apple claiming it to be around 8 hrs (compared to 9 hrs in the 13" model).
The same goes for the VMs. I don't have VMs running constantly but from time to time. Maybe 10% of the time.
What about this:
I buy the mid-range 13 inch model next week - more or less as a temprary solution. I'll try everything out and find out if it's okay for me. In the possible case I find out it's not sufficient for me I'll sell it around this time next year, maybe a little bit earlier. The resale value probably will still be quite okay by then so I won't lose too much money. By doing so I can bypass the waiting time until the Broadwell or even Skylake models come out. The remaining 1000 bucks I "save" by not buying the big model are going to be invested in Apple stocks. So probably I'll be rich next year anyway

. The only downside to this temporary solution is that I will find myself in the exact same situation again.
Btw.: I wanna thank each and every one of you again for helping me so actively. Helping people deal with their first-world-problems is not exactly exciting, so I really appreaciate it very much. That being said: feel free to contribute even more