Both caves are wonderful, so therefore, I'd recommend seeing both if possible. If no time, here are some things to consider:
Is this anyone's first cave tour? If so, I'd definitely do Jewel. It's more open and has the typical stalactites and stalagmites, cave bacon, popcorn, and crystals. But..Jewel's Scenic tour has about 700 steps, which brings me to the next question:
Do Grandma and Grandpa (or anyone else) have difficulties doing steps? If so, you might consider a shorter cave tour at Wind Cave. There's the Garden of Eden tour that's only about an hour and have fewer steps (only around 150). It's considered the least strenuous tour offered there. All the tours at Wind have fewer steps than Jewel. Do keep in mind that Wind Cave is much more closed in than Jewel and might not be suitable for anyone who is claustrophobic.
I have seen both and they are very unique from each other, so there's a huge difference. Wind Cave is known for a rare formation called boxwork, which resembles spider webs. If I absolutely had to choose one, I'd pick Jewel.
As for reservations, definitely make one if you decide on Jewel. You can wait several hours to get on a tour there.
To help you with your decision, visit nps.gov and look at images on both caves to see which formations might interest you more.
Have a great time!