Speaking as 'that guy' I'd say it's really a personal preference here.
I'd go for 16gb ram for starters (not sure if that's an option for the normal MBP) - I've found it makes a massive difference having more apps open - and you will see a benefit in FCPX.
Beyond that, it depends on whether or not you prefer size over upgradability - either are fine machines, but I found the 512Gb flash drive (faster than the 256gb SSD I think) with the 2.6gb retina too good to miss. It means you can even store some broadcast-quality media for viewing/editing - something I'd never otherwise recommend. On the other hand, if you don't care about the extra speed, you can fit a larger HDD in the standard MBP. Not sure I see the point of bumping up the cost of the standard MBP over the 'standard' Flash storage in the retina.
One thing - I'd go thunderbolt for storage, over USB3 - although I use one for transfers to and from other computers.
Either way - you'll be happy.
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