The usual advice is:

1) get a backpack that holds what you need without a lot of extra space left over that will tempt you to fill it up with more stuff you don't need. (NB: 70 liters may well be too big, but I don't know what gear you have, or you feel you "need".)
It is not stupid to bring your actual load of gear to the store to test a pack. that's what you'll be carrying after all.

2) get a backpack that fits you. Bodies differ and what is a great fit for others may not fit you all that well. If the size is adjustable, have an experienced fitter adjust it to you. Then load it with a full load and walk around a while.

3) labels and makers don't matter all that much compared to size and fit (see again points 1 & 2). Reputable makers don't make shoddy goods. No-name packs can be surprisingly good, but learn to look carefully at seams and stress points for signs of poor workmanship. Regardless of the maker.