I'm almost in the same boat. I have an early 2011 17" MBP also with 500GB SSD and 16GB ram.
I, though, won't upgrade any time soon.
Unless you render videos the performance difference will not really be noticeable. The display... well... I'm used to and kinda need the Full HD display of the 17" MBP. The rMBPs default resolution just doesn't cut it space-wise.
If you scale it the quality takes a hit and things border on being too small.
In addition to that these two machines are just too similar.
Both got DDR3 memory. The GPUs are not sooo different performance-wise. Both can drive 4k displays.
Which is why I personally am waiting for a MBP with:
- DDR4 (as it is a lot faster than DDR3)
- a native 4k display so that it can render 1080/1200p at 2x natively (there alrdy are 4k panels of that size... so it's only a matter of time until Apple incl. one
- Thunderbolt 3 with 5k support. Once that comes out, I think it is a pretty safe bet Apple will release a standalone 5k Thunderbolt display!
Which I hope will be realized with Skylake... or with Cannon lake...