spider you can mix with anything, and get spider + what ever it was....
mystic does sorta limit your options on what you can mix it with to get something noticeably different than what ever you mix it with, but then again, when you do mix it with that other thing that it mixes well with, it is really cool!

so, do you want a lot of good breeding options (spider)
or a few great ones (mystic)

at least how I see it

also keep in mind this is a girl, so you will only get one breeding from her a year at most, so you may not need so many options. What I mean is, if you had a mystic boy, you may not have 3-4 good females to put him with. If the spider was a boy, then you can put him with many different females, and not have to be "limited". So being a little limited with a female is not as bad to me.

don't forget to put the most weight on WHAT YOU LIKE! You already said you don't care for the mystic, so it is really just for the potential of mixing with something else (which means that if you mix it with another single gene animal, you will only get about 25% of something you care about). If you like spider, and mix it with another single gene, you will get 25% of something you were trying for, but an additional 25% of spider, which you seem to like more than mystic.

And also, what males do you have? if you have a mojave male laying around, maybe you want the mystic for him??

personally I don't care for spider (just never liked the pattern), and I JUST got a 2014 mystic potion female ($500 at a local show)