I'm personally more worried about the cholla when I MTB, but it's mainly because you can see saguaros from a mile away, but the chollas can be low, lurking around other stuff, and you might not notice them until it's too late. I've so far managed not to impale myself on any saguaro, but I've had some cholla experiences that were no fun. I did blow up a cholla one time with explosives during some military training out in the desert, and there was this one cholla that had encroached into the demolition training pit we were using. I thought of all the times I'd had to use a Leatherman to pull the spines out of my boots and uniform trousers, even when I'd tried to be very careful and keep an eye out for them. There's probably this fan-shaped arc of desert extending out from that demo pit now that's just crawling with cholla, lol.