With any piece of gear, it's always good to ask, "how does this piece work for me and allow me to do the things I want to do?"
That said, I would first look at application. Where will you be sleeping? Are you thinking about using the pad in a hammock as well as on the ground? Will you only use it inside your shelter, or do you need extra abrasion resistance so you can throw it on the ground during a hiking break or in camp? Are you looking for the most well-rounded pad, or are you willing to use a short length pad and put your pack or a Z-Seat under your feet to keep it as ultralight as possible? If you have a well-planned kit already, how does the pad fit into and work with your system like your current sleeping bag and shelter? Is it small enough to fit inside your backpack like this one, or will you have to lash it to the outside somehow as most do with closed cell foam pads? How fast is it to set up and break down, and will you feel like doing that set up and break down every time you take a break or set up camp after a long day?
Then I would look at comfort. Are you a back/side/stomach sleeper? How much padding do you need to be comfortable in the positions you typically sleep and wake up refreshed for your next day? Do you have broad shoulders, or are you tall enough to want a larger size? Do you sleep cold or plan to use it in freezing or near freezing temperatures in the shoulder seasons, and want extra insulation in order to maintain comfort?
Then I would look at other individual pad features. Does the pad have a non-slip coating to keep it from sliding around on your tent floor? Does it crinkle loudly when you move around and wake up you or your mates? Is it flat, or does it have various ridges to keep you on the pad?
I'd also suggest you go to a brick-and-mortar outdoors store like REI and lay on a bunch of their floor model pads. You may be able to figure out some of the features you like and dislike from handling them in person.
Hopefully this gives you a good starting point.