As I've said, I think that any USB3 external will be fast enough. Therefore I'd go for the largest capacity device that's portable, by which I mean (single) bus-powered. At present this seems to be a 2TB, 5400rpm, 2.5" HHD. If this proves to be fast enough, you win.
If this doesn't, then you're going to have to look at faster options (and the only significantly faster options I can think of are SSD, meaning loss of capacity, or proper 7200rpm/RAID0 gear, meaning loss of bus-powered portability). But your 'rejected' HHD will still be very necessary because you've got no backup/archival in place. Again, you win.
Win... win... you can't lose.
But please don't rush out and buy something right now, wait to see if anyone comes up with a counter-suggestion.
As an aside, I'm assuming that you and the rMBP are moving around. If it spends all its (working) hours in one location then obviously 'portability' moves down the list of needs.
Yeah Im moving around alot so it MUST be portable. I think in terms of backup I really cant be bothered and I know ill regret it but i could always buy a 50 quid 1tb WD drive for backups.
However ! This could be the only half decent idea I have and I think this product maybe perfect for me.If there are any others like these or better/cheaper Id love to know but this could be the one!!
http://www.wdc.com/en/products/products.aspx?id=1240
But of course the verdicts down to you