I think I can offer you some insight.
I live SoCal and drive many different vehicles. IMO 18" is the right size for the X1 here. I just sold my e90 335i sport with 18s. Moving from the Bridgestone RE050a RFTs to Michelin PSS transformed the car in both ride quality and performance.
My wife commutes to LA everyday. She has a 128i sport with 17s and trading the RFTs for some cheap Hankook V12s made a huge difference (she loves the car now, not so much with the RFTs).
I used to drive an E39 540 (Eibach ProKit and Koni FSDs) with 19s. I had a few different tires on that but the best ride quality was with Bridgestone Potenza 960s. I hear the 970s are even better.
I'm planning to go with Michelin Pilots or the 970s...or perhaps something better will be available, when the time comes to replace the OEM rubber.
Conventional 18s will ride better than RFT 17s.
Also, we carry a donut for emergencies vs a slime kit. Never had to use it (knock on wood). We just drive to our nearest America's Tire when we have a puncture.