It depends what you mean by Ride Designer. You could either mean a creative designer, or an engineer. I don't really know of any roles in serious attraction design that would be purely creative other than theming and marketing, like Candy Holland does; it seems that all 'creative designer' roles have an element of engineering in them, as with Disney Imagineers or John Wardley. Having said that I've never looked into the purely artistic side, I may be completely wrong.
Whilst both sides are obviously very hard to get into, I'd imagine the engineering side gives a better range of options if the plans fall through. I'm doing an engineering degree, originally having been inspired by parks and coasters at the age of 10, and although it's still my dream to design them, I fully appreciate that even with a hypothetical really good degree there's a 99% chance I won't get to do it just because the industry is so small. But I know that there are plenty of other fields in engineering; energy, transport etc. that I'd love to work in. Plus banks/investment companies apparently like engineers because of the obvious numeracy involved, although I've never really looked into that either. But even if you mean purely artistic I'm sure there's a good range of options like product design and that kind of thing.
Acting seems much tougher in terms of options; of course there are many different mediums (Film, TV, theatre) and many tiers within those media, but still you're doing the same thing in every option, and as the competition in all of them is insane it would be hard to fall back on anything else. Obviously drama teaching is a possibilty, and I don't know what kind of demand there is for teachers.
I'd say ride designer, mainly because I'm biased and that's more the kind of thing I'm hoping to do, but try to keep both options open as long as possible; it should be doable to keep both going (maybe not doing degrees in both, but then again even that can be done) especially if you're genuinely interested in both of them.