Here's my dillema. I have a set of primes: 35mm f/2, 50mm f/1.8 (Mark I), and an 85mm f/1.2L II USM. I didn't have the 85 or my 6D with me yesterday, but it was nice out and I was off work early, so I figured I'd grab my EF-S 55-250mm f/4-5.6 IS II and slap it on the T2i, since I didn't have my 85 to put on the 1DIIN. All that playing with a maximum 35mm equiv. of 400mm got me missing the reach of a nice telephoto.

I've been playing with TheGirlfriend's 70-200mm f/2.8L USM whenever I got the chance, but that led her to remembering how awesome it is. So now, I'm going to have to get my own telephoto. I really don't find 200mm to be nearly long enough for my 6D, so I'm looking at something between 300 and 400mm, which is the max length of the nifty 250 on a crop body. I figured that I'd use it quite a lot (I live in a corn field, things are far away), so I am primarily looking for a prime tele lens.

I've narrowed it down to these:

Canon EF 300mm f/4L IS USM

Canon EF 400mm f/5.6L

They're both the same-ish price. One's 100mm shorter, but a stop faster and has IS, but the other has a 100mm longer reach.

What would you do?

Note: Equiv lengths on my cameras: [6D/1DIIN/T2i]

300/390/480

400/520/640

I don't plan on carrying a tripod everywhere.

If there's a different lens you'd suggest that's /around/ the same price, let me know!