If I was in your position would have kept the 2015, "if" Apple redesigns the rMBP on Skylake`s release it may not be all good, new product, new problems and Apple`s infamous Gen-1 experiments. Sometimes it`s better to buy end of line as all the engineering issues are solved and the product is extremely stable.
If Apple does redesign the rMBP you can guarantee it will be thiner, the thermal envelope will be pushed harder, USB C is a given, they may up the resolution to 4K and crucify the GPU further, and likely not optimise OS X for it. Being straight faster hardware is useless if the TDP is exceeded and Apple`s focus is aesthetics first we all know this. I am considering upgrading my 2012 Retina to the 2015 2.5 for the same reason, stability & performance, equally I have time on my side as my 15" is still a fast portable and I also have high end 13" Retina MBP & 12" MB.
All very much depends on your needs & use, think your 07 MBP has probably run it`s course, same as my Early 08 MBP did, and I retired that in 2011
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