I don't know why people are saying skip Barcelona. Even though it has tons of tourists, they can be navigated around. Just avoid Las Ramblas and 90% of the tourists and pickpockets are not an issue. There's some AMAZING architecture there, and not just Gaudi - take the tour of the Palau de la Musica Catalan - one of the most beautiful concert halls I've ever been in.
Some years ago when I was younger and more energetic, I brought a whole Hasselblad kit with me to Spain- 500cm, 50, 80, 120 lenses, a Hassy Superwide, and at least three film backs. Oh, and a Fuji TX1 (aka Hasselblad Xpan). Today, for multiple reasons, I'd take my pair of Rolleis and my Lomo LCA 120 to cover the wide-angle end of things. Or just my standard Rollei, which really is sufficient for the majority of images I'd want to make (I took just a standard Rolleiflex 2.8E with me to France in 2015 and shot some of the best photos of my life). I've also done that in Italy with the 2.8E, the Tele, and a Lomo Belair X/6-12 (for panoramics). So, in regards to your question about 35mm vs medium format, I would take the medium format with you without hesitation. Spain is fairly camera-friendly and certainly highly photogenic. I think you might regret not having the bigger negatives down the road. But then I'm also a little crazy- I took an RZ67 with three lenses (50, 110, and 180) to Mexico and shot street photos with it.