Go with a .270 win - you'll have lots of loading flexibility -long flat shots or big game loads- Great sectional density. The best part is if you get a nice Kimber (one of the nicest light rifles on the market under $1K, it won't kick the snot out of ya. ) My buddy just bought a ultralight weatherby 7mm (6.25 # w/ scope?)- it will knock your teeth loose. Remember as weight goes down kick goes up.

Leave plenty of $ for optics- just as important as gun for plains.