Firmness changes the horn more than size does. A firm silicone unicorn gives you a defined ridge on every single stroke. A soft one smooths that into a gentle wave you barely register. Decide which of those you want before you start thinking about inches, because it makes a bigger difference to how it feels.
Glass sharpens the texture considerably. The same spiral in borosilicate reads much more pronounced than it does in silicone, and it holds temperature, so you can run it warm or cold. Excellent, but not where to start if this is your first textured toy.
If it is a gift, the colorway is the decision. Nobody choosing a rainbow unicorn is optimizing for length. Pick the one that looks like the person you are buying it for and you will be right more often than you would be agonizing over specs.
Check the base if you want hands-free. Most of these have a suction cup or a flared base, which means they stick to a smooth surface or drop into a harness. Worth confirming on the product page rather than assuming.
Glow needs charging, and that is not a gimmick. Ten minutes under a lamp or in daylight and it will glow properly. Straight out of a dark drawer it will barely do anything, which is where the disappointed reviews come from.
Water based lube on the silicone. Silicone lube permanently degrades a silicone toy. Glass will take anything, but water based keeps it simple across the whole collection.
Anything anal needs a flared base, not a suction cup. Those are not the same thing. The swirl plugs and tails are built for it. A unicorn horn dildo, in most cases, is not.