To be fair, I don't know anything at all about how their (1) their features, or (2) their support are since I fired Grade.us and switched to GatherUp in May 2018.
What I can say is that at that time, a year ago, support at Grade.us was bad (as mentioned above) and the features were, well, not very easy to use, for the most part. Composing the review request emails was very difficult, with a quirky UI that often lost all the work I had done and also arbitrarily removed things that I wanted left in, but I cannot remember now what those were. One thing that I DID like about Grade.us, that GatherUp does not yet have, is automatic sharing of favorable google reviews out to Facebook, but in the overall big picture that's a relatively minor thing.
I also really respect Mike Blumenthal's experience and expertise and so that is another reason (for me, at least) to lean in favor of counting on GatherUp for the future.