Well, the way I see it is that Star Trek is outside my comfort zone in multiple ways. For one, there's the 2d20 system itself which has a bunch of stuff I find interesting but somewhat alien to both the way I and the rest of my group are used to playing. And in addition to that, there is the whole thing about it being Star Trek, which kind of requires a different kind of adventure than we are used to as well. So the Star Trek RPG is "strange" to me in two different ways. It's an exciting kind of strange, but still strange.Why? Star Trek as an RPG has been around since the 80s. Just go for Star Trek.
Infinity, on the other hand, can work with fairly traditional adventuring styles. I mean, ideally not dungeon crawls, but more investigatory and/or action-adventure type of things. So that lets me try out the 2d20 system in a more familiar environment to see what makes it tick and how the whole momentum/threat thing flows.
Basically, when doing science you want to change one variable at a time in order to see what effect that particular variable has. If you want to figure out what sort of things affect the current in a wire, you don't want to change both its thickness and its length at the same time. You first keep the thickness constant and try different lengths, and then keep the length constant and try different thicknesses. Changing both at once means you don't know what part of the effect is the result of which change.