Should one wait for new Mac Pro? I want to buy Mac Pro. I've found used 1 year Mac Pro with Xeon 3.5 6-core with 2xD500 which is cheaper around 1700 euro then new one. However knowing there's new Mac Pro coming (hopefully) this year I wonder if I should wait. The question is about potential new Xeon and new GPU and how much better in terms of performance it is comparing to old Xeon. Lately all the changes in CPU/GPU are to make them less power hungry rather than having more power so the changes aren't that big. What sort of power difference can one expect comparing new Xeon to the old one?
TLDR; If you care about your money, buy that 2nd hand MP.  If you care about expandability, look for a 2nd hand heavy CPU'ed cMP (classic Mac Pro).

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That used Mac Pro doesn't sound bad at all!  Sure, one day in the future there will be new models available, and they will be faster and better.

However one must not buy a Mac Pro - or any computer for that matter - for the future.

Assuming your computer is not for academic research or playing games, professional machines like these should start making you money from the day you connect them to the power grid.  When the new model comes out you should have made so much profit with it that replacing it would be a no brainer.

If you don't plan to do that - and are afraid it will be quicker obsolete than the upcoming version - perhaps the Mac Pro isn't the machine for you.

Chances are low the 'new' Mac Pro will follow the latest traditions of the 'lesser' Mac range, being getting smaller, less power demanding and just a slight faster or less fast than its predecessor.  The new Mac Pro will be significantly better leaner and meaner than the Late 2013 model.

Expect the new MP to have a E5 V4 Broadwell-EP - or even a Skylake E3 V5, Thunderbolt 3, USB 3.1 and an updated AMD FirePro or even the Fury.  These are very cool features indeed but IMHO the steep price and limiting form factor make it not worth it.  Give me 2 grand and I'll search myself a 2nd hand 12-core, matte silver boxed classic Mac Pro.

disclaimer: personal opinion of course.  We swapped our 6 core cMP with a 6 core newMP about a year ago.  If I knew better then I would have tried to stop that.  That's for technical and usability reasons, not for the (companies) money.