What's open field, or what range are we talking about?
I'm a huge fan of the .270 Win! If you reload, use a fast twist barrel, and take advantage of some of the newer bullets it will do the job very well. My longest range mule deer was taken with the .270 and a 130 Sierra GK just under 600 yards.
Honestly, until you get to extended ranges there will not be a noticeable difference between a .270, .280 AI, or 6.5-284 on game. If you just want something different, then I think they are both excellent choices. I spent a lot of time trying to find a better performer on game than the .270, mainly playing with the 6.5-06, .280 Rem, and 7mm Rem Mag. I still have a couple .270 Win rifles and I've sold everything the others mentioned off.
Have you looked at the .25-06 for an open field deer rifle? It's a pretty good choice with a long action build for a deer rifle. If I hadn't built mine so heavy I'd still have it. It was a great rifle if you hunted from a blind, but it sucked as a mountain rifle.
500 yards would be max range I'm hopefully joining a hunting club in SC and I can shoot past 500 yards on some power lines areas.