I would think about what your intended use of the sights is. If you're using them as primary & need absolute accuracy at distance then go with a barrel mount. If they're backups & you're using a quality optic that's not likely to go down then I don't see a problem with mounting them on a free float forend. If you're talking about a snap-in rail system then I'd definitely stick with the barrel mount. You can press on the tube hard enough to create some deviation but it's not going to be nearly as bad as a tight sling on a standard barrel mount.