.243 Win. may be a tad light for a big mulie at 500yds, but perfect for Antelope IMO.
I like the .270win over the .25-06, but that's just me.
6.5creed, or 7mm-08 options to consider, I'm thinking about a 7mm-08 lately, but no plans yet, I like the idea of a 7mm08 in a 20-22" barrel.