The big difference between the two for me is how much feeling of gameplay expression there is. In Fallout, my options feel like melee, shooting enemies with shotguns, shooting enemies with automatic rifles, shooting enemies with long-range rifles, shooting enemies with lasers, shooting enemies with miniguns, and so on. And the shooting mechanics don't feel strong enough to really differentiate those different weapons as different playstyles for the most part. If I play a game like Titanfall, Battlefield, etc, then changing weapons can feel drastically different - they handle differently, you navigate combat arenas differently, you prioritise targets differently, you use cover differently. But that doesn't really feel like the case with Fallout for me without any of the moment-to-moment decision making that tends to allow for gameplay expression in shooters.
Whereas Skyrim feels like there are a lot more playstyles available. Destruction magic feels very different to conjuration which feels very different to illusion which feels very different to being a stealth archer which feels very different to using a dagger which feels very different to using a huge, two-handed melee weapon. They're not just visually different; how you approach and navigate combat encounters will be significantly different depending on what kind of build you have. It just feels like there's so much more gameplay depth.