I went with 64GB for mine and there have been a few times when it really did come in handy. When working on large projects in Motion, AE, Smoke or FCP X I regularly see the available RAM drop down to below 32GB, especially once I'm in the finishing stages and using a lot of FilmConvert and Neat Video stacked with BCC etc. I tend to finish in FCPX so they'd be the FxPlug versions, not OFX. So far as Resolve goes, I don't think the application's ever used close to 32GB of RAM, even with 10-20 active nodes. I don't have access to Nuke on this machine, but if Motion and AE are any sort of precedent, that's where you're going to want the 64GB.
FCP X Certified Pro, Level Two
Traditionally RAM usage for Nuke has been as much as you can throw at it, since it uses RAM for caching on playback.
However, with the boot drive now offering very fast read write times (~1000MBps) disk caching is a viable option and can actually provide decent performance.
Using the internal boot drive for disc caching is sort of a poor mans FusionIO card.
Thanks for the feedback re Motion AE and FCPX. I don't use those, but I don't doubt that if I have Nuke and Resolve open at the same time as well as some OFX plugins grabbing their own share of RAM, that 32GB might get used up.