I’m thinking of buying one of the two, which would you buy and why?

What are your plans with the rifle?  7mm Rem Mag brass & ammo is much easier to find, and bullet selection is significantly better than the .270 WSM.

7mag

140class bullets for medium game, 160-195 class bullets for big game/ ELR. You may need a custom barrel to shoot the 180-195’s (1:8.5/1:8t), but they have a very high BC.

I don’t reload. What’s the heaviest bullet that I can buy over the counter that’ll take large game?  Does 7mm or 270 wsm have the heaviest?

7mm will have the heavier bullets.  You can go up to 195gr bullets with the 7mm.  Berger has a 195gr EOL, but you need a 1-8.5 or faster twist to run it.  Browning makes their X-Bolt with a 1-8 twist, and Ruger makes their M77 rifles in 7mm with a 1-8.5 twist, so both would be capable of running the 195 grainers.  A few specialty ammo manufacturers load the 195gr EOLs.  Hornady makes a 190gr A-Tip match but they recommend a 1-8 twist, and I am not sure if there is anyone factory loading it for the 7mm.

7mm Rem Mag, all day long....I wouldn't even consider the .270 WSM if I didn't reload.

7mm bullets are slicker than snot.

Chris