I went from t3i to 5d3 and it is night and day. The AF, build quality, LCD, viewfinder, high ISO, button customization, wider FF shots, 1/8000 sec shutter speed, 6 fps, 23MP, HDR.... list goes on.
I do miss my swivel screen and pop-up sometimes though. Had to buy calumet wireless transmitter.http://www.calumetphoto.com/eng/product/calumet_pro_series_2_4ghz_4_channel_wireless_trigger_kit_optimized_for_nikon_flash_systems/cf0094
There are things no one is discussing. The viewfinder is 1.6x bigger and brighter. The button customization is amazing. Now I can change ISO, aperture, & shutter speed so much more easily. It makes manual mode much easier to use. T3i is max 1/4000th second. So on bright days, you might not be able to open up to f/1.2 on your 50L.
ISO 800 on the t3i is like 25000 on 5d3