Picking a uni is always a nerve-wracking decision, even when you're sure about where you wanna go. If you haven't already, visiting imperial may give you a more balanced perspective. Maybe see if you can talk to anyone you know who has been to those unis.
Ultimately, I think it's best to go with what your heart says. If you don't, you may be in a constant state of longing and wondering throughout your whole degree. In the end, you can't really know for sure how great the uni experience will be for you until you actually go there and start your degree. All you can do is go with what your gut is telling you!
Official Student Representative for the University of Southampton