My Eos 5D gets chosen automatically unless I'm covering "events" or bird photography, when I'll take my 1D MkIII. (I also have a 30D and a 10D in the cupboard.)
Visiting Manchester with its Beetham Hilton Tower last Tuesday, there's a mass of shots I wouldn't have been able to get to the high quality I wanted, except with a full frame sensor. I mainly used my EF 50 f1.4 at around f5.6 or f8, and for my style of architectural record, the detail from edge to edge is most gratifying. (I had my EF 24 f2.8 in reserve for wide-angle shots.)