is the 6.5x55 commercially available? isn't the 6.5-284 still purely a wildcat? though semi popular? if both were wildcats I'd pick the 6.5-284, but if the other is factory loaded, that would be a big influence, but i know many load the 6.5-284 for 1000yd benchrest......if both are known for accuracy and the x55 is factory loaded again be a big plus IMO. otherwise id take the -284 sine I hear more about it.
Now this is just on cheaperthandirt.com but, lets see they have remington, winchester, federal, hornady, wolf and a few others, yep it's available, but most sporting goods stores aren't going to carry it.