Magnets really are far and away the best approach if you're going with a faceshell. If you go with glue, you've "married" your lens to your mask and that means if you ever decide to upgrade/replace your mask, you have to replace the lens too (or go through the messy task of tearing the lenses off the mask, leaving you with lots of hardened residue on the underside of the lens that'll be a nightmare to cleanly remove).
But a magnet/faceshell combo means you can switch out masks with ease (as I've done, going through numerous mask prototypes). It also allows you more freedom to re-adjust the placement of the lens on the faceshell if you wanna make them higher/lower.
TIP:
1. Glue rare earth magnets along the underside of your lens (E6000 works great, but the fumes bother some people; I'd imagine hot glue would be a good choice too). The flatter the magnet, the better, but don't go *too* small or the holding power will be weaker.
2. Once the glue on the lens is dry, hold the lens against your faceshell and find the best position. Holding the lens in place with one hand, take some loose magnets with the other hand and put them inside your faceshell until they "find" their mates and click in place. Now you can move/slide the lens around on the faceshell until you find the perfect position that you're happy with. Once you're happy, you can dab glue over the magnets inside the faceshell to hold them in place. You can also tape them into place with packaging tape if you want a less permanent placement if, say, you still haven't made your mind up for the final placement.