RE: Remington Model 700 .270 or Mossberg 100ATR .270
The Remington is by far the better rifle of the two you listed.
Go to a shop and look at them both side by side and examine the fit and finish, smoothness of the bolts, machine work, etc... etc... andI think you will see the Remington comes out on top.
If you want to get a good accurate rifle but spend about what a Mossberg will cost then look at the Stevens 200 or Savage 110.