Hi!! First of all well done at getting offers from Cardiff and Stirling. They're both great unis.
I am actually typing this as I wait for the train back to Stirling! I really love it. If I can describe Stirling, it's a pretty sleepy town with not a whole lot going on. There's only one nightclub (FUBAR). Personally I'm not into drinking or clubbing, but if you're looking for a party scene I wouldn't recommend Stirling. It is not a huge party uni. The town is nice though in that there's some really good restaurants and it's quite a "studenty" area. Bridge of Allan, which is just a little walk away from campus, is also incredibly beautiful. There's multiple cafes across campus, food served at the union and a restaurant just outside the entrance of the uni so you're never stuck for options
The campus is so beautiful. It's especially gorgeous in the summer. Stirling is a really great choice if you want an on-campus uni — it feels very safe and you've got great connections to everything. The facilities are great too. The library has different levels: level 2 where you work in groups, 3 where you generally work in silence, and level 4 which requires total silence. Free period products are supplied in every bathroom on campus. The gym has some really excellent facilities and you can book workout classes, like spin, dance etc. You've also got a Co-op, stationery shop and cafe right over the bridge (all located in the Atrium) as well as the Student's Union right beside it. Classes are held mostly in the Cottrell building which again is right over the bridge and super easy to get to. Buses come right onto the campus which means it's very easy to get to Glasgow, Edinburgh etc. Just get the bus into Stirling and then the train to wherever.
I find the campus atmosphere very welcoming. It's a smaller campus so you don't feel as "lost" as I suspect you might feel in bigger unis. There's a good amount of societies too and it's great for sport clubs, as it is a sporting uni! If you're interested there's also a lot of Christian groups and Stirling has a large and active Christian Union. There is also a chaplaincy. It's good for international students too, with a "Global Café" on campus where international students can meet and specific societies for international students eg the Nordic society.
If you have any kind of learning or developemental disability there are places on campus where you can get support for that. I haven't personally used these resources but they seem quite good. You can also get support for any mental health issues; the uni offers free therapy sessions and there's a student-led night time hotline called Nightline.
I like the classes too. I love Scotland's system of choosing optional modules to go alongside your main topic of study. For example I study psychology but in my autumn semester I took additional classes in religious studies and sociology. It makes it much easier to switch your course.
I really enjoy Stirling. I chose it over Cardiff because I've heard Cardiff is a pretty big party uni as it's a large university too. I didn't like the idea of living in a big city and I preferred the "security" of Stirling as an on-campus uni.
I can't tell you too much about Cardiff as I obviously haven't been there, but I really recommend Stirling. I tried to put as much information as I could here to help you, but feel free to ask me more about specific things
Also if I could tell my past self anything — the ranking of a university does not matter as much as you think and you should choose what's right for you.