There's some great anecdotal reports all over this forum, just look around, but if you're into benchmarks this page is great:
http://www.barefeats.com/imac5k6.html
Obviously the biggest misrepresentations you'll find in the forums are around how unacceptably 'slow' and 'outdated' the M290X GPU is, the benchmarks add some good perspective
It's always a personal decision whether to take a fast machine even faster for an additional $500. Just don't do it for the sake of the "future", since both versions will be slow in comparison with entry level machines in 3-4 years.