Given that you already own a .308 Win and a .30-06, is not even close in my book.
The advantage the 6.5 has is that it will handle heavier bullets – up to 160g is available from Hornady, but 140g is more typical. But you already have two rifles that handle 150g bullets very well.
In the middle weight, they both push a 120g bullet about 2900fps.
On the lighter end of things, the .257 handles 75g bullets very well and can push them to 3600fps. The lightest bullets for the 6.5x55 are 95-100g and get pushed to about 3100-3200fps.
I’d go with the .257 Roberts. Conley Precision Cartridge loads the Hornady V-MAX to 3600fps, which duplicates my load for the same bullet. I’ve used my 75g V-MAX load to take a coyote at 480 yards and many at shorter distances. The Roberts and the V-MAX have become my favorite varmint combo. My 120g A-Frame load steps out at 2947fps (a +P load) and the Federal 120g Partition load does 2807fps from the same rifle.
If you can’t get it done with the Conley or Federal loads you should be considering something other than a .25 or a 6.5. Like your .308 or .30-06...