Advice on how to shoot people and horses (portraits)

Tomorrow I'm going to be taking some shots of one of my good friends and her horse. I thought this would be a good idea for my "Environmental Portrait" assignment for my photojournalism class.

Equipment:
Canon 50D
17-40 f/4L
24-70 f/2.8L- unable to use tmrw
50 f/1.8
135 f/2L
580ex II- omnibounce

Which lens should I use?

so what are some basic settings for inside the barn? then some settings for outside? What should I be looking for?

I think probably the 135 - or maybe the 50 for some detail shots?

Careful of the flash, it could make them spook...I imagine the barn might be dark, so think of outdoor portraits.

ETA: Look in the Pets and sports sections for some equine photo examples