I have a 25 wssm in a heavy bolt gun Winchester. I also have a stockpile of brass stashed under my bench to keep it running. I think the day will come when the brass is history. That being said, when that day comes, I'm sure forming dies will come out to form it from wsm brass, which is the parent case anyway. Whichever one you pick, grab up as much brass as you can, preferrably Federal brass over the Winchester stuff.
You will find a much wider selection of varmint bullets in .243 than .257, so depending on how important the varmint side of your plans are, keep that in mind. My rifle REALLY liked the Berger 110 gr MEF varmint bullets. Sadly, that bullet is discontinued, per Berger, the 25 cal varmint bullets were their least popular sellers. When my current stockpile of bullets is gone, I have to decide of I want to try another bullet, or rebarrel my rifle to either 243 or 264. Any bullets I've tried in the varmint construction have all been too light IMO for the long range task I bought it for. My gun likes the 110-120 grainers the best, but the 115 Ballistic Tips tend to just pencil through the prarie dogs at medium to long range.
Of course for deer, the field of 25 cal bullets is more than adequate to the task.
So, it depends on how much varminting you plan on doing with this rig?
I think I would lean towards the 243, and with proper big game bullets, I'm sure it'll be dandy on deer.