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Sounds like the 5k will be the best choice for you - there might even be a cheeky update of them by the end of the year so keep your eyes peeled.
As much as the 5k screen sounds amazing I was worried it would kill it as a gaming machine - downsampling can have mixed results. Maybe I read too much about thermal throttling too, I think the next batch of AMD cards and Intel CPU's might fix those problems though.
Ta for all this. Good for thought. Though still might opt for the retina version simply for more screen real estate. we work with 4k video a lot but not enough to justify a set of Mac Pros and 4k monitors. The iMac 5k seems a good compromise.