I've noticed at all my local gun dealers that they keep very few new 300 Win Mags in stock anymore and those collect dust, but the 300 WSM are plentiful and the guns fly off the shelves. Maybe the 300 Win Mag is slowly on the way out?
I've seen the same thing in WA, OR and CA. My brother lives down in NM and he tells me the .300 WSM is the most popular cartridge by a wide margin (for 300 mags). I don't know if the .300 Win Mag is on its way out...it still makes most top seller lists...but I can guarantee you its sales have taken a hit and the .300 WSM is here to stay.
That being said, I always look to the biggest variable of all when I'm comparing cartridges: Recoil.
If two cartridges generally carry the same performance, I always go with the one that has the lighter recoil. It will be easier to shoot, more pleasant to practice with and will enable you to get to where you're good enough with itto take that 500 yard shot sooner than the heavier recoling cartridge. That alone would make the decision easy for me, as the 7mm WSM is for all intents and purposes the equal of the .300 WSM on game
However, if you go with the 7mm WSM, then a very logical progression of thought would have to consider the .270 WSM. It recoils slightly less than the 7mm, its performance is the equal of the 7mm on game, and ammo is more widely available.
If you're dead set on a shorty in either the .300 or 7mm, I'd go 7mm. If you still have an open mind after that, I'd go .270 WSM.