Just to close the loop on this -
As stated before, I went with 1TB drives but I'm replacing the 500GB hot spare in my other shelf so I can have three 4+1 R5 groups instead of two 6+1's. I'll end up with the same space (12 drives worth after RAID overhead). For the extra bucks, the shorter rebuild times and the common remark about the Celerra working "mo betta" with more RG's I feel it was worth the (relative) small expense.
Thanks for all the comments and words of wisdom.