At the surface level, a lot of it comes down to specs-
Here's
some info on the TX1 -
and
some on the TK1 -
That's probably the closest I can get you to a direct side-by-side comparison.
Which one your prefer comes down to how you plan on applying it. I've never used a Jetson as a coprocessor on a FIRST robot before, but I've used both in the past and am personally a huge fan of the TX1- it's fast, pretty easy to access/use/set up, and it's very powerful. The TK1 might be more appropriate if you're just using it for computer vision or something though.