For my weddings I shoot with a 5D and use my 50D for a back up.


On the 5D I mount my 24-70 f2.8 and the 50D has the 70-200 2.8


(some would switch that but I use the 5Ds ability to shoot higher ISO and wider for inside the church and the 50D is mostly for outside shots assisted by the 5D 24-70)


I want to have a third camera just for a 50mm 1.4 for those close in small DOF shots.


I dont like changing lenses and settings during the shot groups if I can help it.


I want to keep the 50mm 1.4 (or a similar lens) set at A 1.4 T 100 (manual)


I need the 1.4 for the shallow dof and the 1/100 to ensure to compensate for the camera motion as these are usually done in a rapid sequence with me moving around the subject.


These are usually some of my best shots.


So here is my dilema:


What to I choose for a third camera (cost is a factor)


Old canon 20d (nice fps, quality is ok, available used for approx 300-350)


Or here is the real question


Can I mix (yes I know the lenses wont mix) can I mix Canon and Nikon


as the Nikon 3100 at 549 (body only) looks awful tempting


Can a Canon user use Nikon or will I be deported?


Other suggestions?