I think it mostly depends on what your MIL's family mostly has. If you're an Apple crowd, I'd go with an apple product. If you're an Android crowd, I'd select an Android tablet. It's the family that will need to help her get up to speed and give her support when needed. And you'll want to be able to communicate with the family using things such as Facetime - the Android equivalent is Google Talk, I believe.
As to what to look for in specs, I would start looking at the specs for some high end tablets and compare them to low end tablets to see where the differences are and try to see what might be of most value to you/MIL. Our kids are getting tablets, and battery life was really important. Storage might not be a big deal if she's not going to be loading movies on or downloading games that require a lot of storage. The other thing to consider is whether she has wi-fi access and would that be sufficient or does she need a data plan. The latter will drive the cost up both on the tablet itself and on a monthly basis.
There are a lot of sites out there that do side-by-side comparisons of the various tablets, so you might look for those. I would definitely stick with a brand name and read reviews (cnet, amazon, etc). I would second Gamma G's Nexus tablets, and at $149, that's a total steal! Those are the ones I got for my kids from Google but at $199 each. I wouldn't go lower than $150, unless that's the sales price.