The advantage of flexwatt is the flexability (lol! No, not literally. OK, well sort of. But they're both flexable in that sense. I digress...). You can do whatever length you need, and have 4 different widths and even more wattages to choose from. It's also a bit cheaper. The down-side is that it needs to be wired by you, although this is not a problem for many people, and some vendors will wire it for you if you ask.
Ultratherm, while a bit more expensive, is sealed (i.e. water-tight), and comes prewired. Down-side: limited lengths.
There may be more, as I have not personally used ultratherm, but those are some general ones I came across when I was deciding between the two a while back.
I went with flexwatt
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