For strops, leather is not leather as you start using certain types of finishing stones.
One thing to keep in mind when looking around is whether the strop is "oil-tanned" versus "veg-tanned." I'm not really a super expert, but the way the leather is cured makes a pretty big difference on the feel and way that it works against your razor. Oil-tanned leathers will have a short break-in period, tend towards a heavier draw, while veg-tanned tend to be drier and stiff at first, but slowly soften and break in with a lighter draw. For beginners, go with oil-tanned for sure.
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Take this all with this understanding that strops are a natural product and grain structure and quality to the strop can and will vary widely.
There's latigo, which thankfully seems to be falling out favor. It has severely extreme "draw" (resistance, drag, pulling sensation) and at best does a "decent" job refining your edges. It also seems to be one of the cheaper cuts of leather, and grain pattern can vary a lot.
There's English Bridle leather, which is about as nice as you get with cowhide leather. It has a lot of oils in it right from the get-go so the break-in period isn't as important as other varieties of leather (you will actually notice that your edges will look a little "cloudy" after use.) It refines your edges nicely, and is usually the style of leather that will work for most folks. I tend to get the "sharpest" and most aggressive edges when using strops of English Bridle. It has a medium-light draw at first, but breaks into a solid medium draw over time. Recommended brands are SRD and TM for me, with TM being the superior of the two on account of the slightly lighter draw.
Then there's your horsehide leather, which can be a step up for some folks. Horsehide has a tighter grain structure than cow hide, so to put it into terms relating to honing, it has a "higher grit rating." There's a few varieties, but I really like the oil-tanned versions that TM and Torolf of Scrupleworks put out. It has a "magnetic" feeling, medium draw and I feel a big difference in how "smooth" my edges feel. They don't feel as sharp or aggressive as after I use English Bridle, but much more comfortable, which is what I'm all about.
Top of the heap is your shell cordovan (different than just corodovan.) It comes from a very small section of the horsehide, it is expensive, and it doesn't even feel like regular leather. It feels almost "stretchy", but the edges are smooth, keen and incredible. Draw is extremely light so it can take some getting used to. I have one from Alastair at Westholme which I love the edges from. I try to really baby it since I have gone through strops due to stabilizers and jimping making scratches and such.
And then you have your odd ducks, your novelty leathers. I haven't tried many, but this category includes ostrich, kangaroo, etc. They work, some like them for travel and such, I would just generally point people towards things that have more reviews on them.
