(2x24-inch or 1x27-inch)+MBPr 13-inch

Okay

I am planning to get a 13inch MBPr, but was wondering if I should go for a dual 24-inch Dell Ultrasharp monitor setup or go single Dell Ultrasharp 27-inch (or a 27 Thunderbolt display, that is a discussion for later)?

I do the odd 3D Design, programming and use lightroom (average at once each per week)

Any suggestions?