Which way would you go and why?
700.
Reason .. the 6mm and .257 Roberts are based on the 7x57 case and depending on how the gun is throated, you may not be able to reach the rifling with a short action and stay within mag length. With a 700 you can install a Wyatt's long mag box which will move you from 2.81-2.82 inches max up to 2.97 inches max. That won't work in a Seven because the additional length in a 700 comes from the back of the current mag well to the front of the trigger. That space does not exist on a Seven action. If the rifle is throated short, then either will work about the same. I do very much like the Seven actions other than mag length constraints even beyond what a short 700 has. With the Seven, you would have the option of having a 'smith set the barrel back a couple threads and rechamber with a reamer with a short throat.
Some years back I had a 6mm Remington AI reamer made. My dummy cartridge for positioning the throat was done with a Hornady VMAX, 75 grain I think, seated to 2.95" since the rifle already had a Wyatt's box from a previous project. Well, the VMAX has a shoulder fairly well forward. When I loaded ammo with more streamlined bullets, I couldn't reach anywhere near the rifling and still mag feed. Sh*t.
Self inflicted. My gunsmith buddy still has that reamer. I've thought real hard about picking up a 700 long action donor and doing the 6mm Rem AI again but right now brass availability for 6mm Rem or .257 Roberts is a problem. I've got the tools to make it from .270 or the like but it's a lot of work to make varminting quanties .. not bad for hunting predators or deer. I decided to simplify and move down to .243 Win. I'm going backwards in 6mm .. my first was a 6x.284, then the 6mm Rem AI, and finally .243. (Maybe a 6x45 someday? :))
Anyway, I guess for your question it depends first on how the rifle is throated. If it has a short throat and allows you to kiss the lands within mag box length limits, I'd rather have the Seven. I'm just skeptical that the individual rifle is going to meet those criteria.
