I'd say it depends on what you want it for. I'd bet almost anything that the 2015 rMBP will have more ports/connectivity options than the 2016 model.
Speed wise there won't be much of a difference. Battery life will likely be identical, given they are going to reduce the casing size (So the skylake power improvements make up for a smaller battery footprint.) I would bet that storage options are cheaper, so you will likely get 256GB in the base model. I'd also bet there will be display improvements - possibly improved color gamut like the 5k iMac, maybe even an increase in size/resolution (But these are just guesses, given a body redesign only happens every 3-4 years.) Possibly true tone like the 9.7" iPad Pro.
The keyboard will likely be updated as well to be more like the newer Apple wireless keyboard.
But either way you should probably wait as the old models will get discounted (And if you buy refurb you can get an even better deal on the 2015...)