I have a Red Sea, but I think if waterbox existed when I got my tank, I may have gone that direction. Waterbox is very active on the Reef2Reef community, and they seem to listen to all the complaints of Red Sea owners and make the changes to their tanks. For example:
Red Sea ATO "systems" are useless, yet they're a big part of the sump design. So Waterbox uses a whole third party system and a large reservoir. For me, I'd still just use a Neptune ATK or something else anyway, so not a huge deal.
Red Sea tank dimensions get a bit narrow when they get longer, they stay about 22" whereas the Waterbox get bigger in width a bit more, so it feels more boxy and less of a wall.
The Red Sea herbie drain valve sucks pretty bad. Waterbox uses a much higher quality one that I heard is easier to work with.
Red Sea plumbing is simple, but generally sucks in terms of how it's attached to the tank and has some super non-standard sizing. Waterbox went all out to incorporate mainfolds and such that you can use.
They both have Peninsulas, but they both miss out on the smaller peninsula tank market. Not sure why... that's the only thing they won't listen to users on. But maybe most people want gigantic tanks. But of all the feedback I've seen, this is the one they have constantly ignored.
The prices are comparable. Waterbox sometimes has a sale (I think they just had one last week), whereas Red Sea tanks are never on sale.
Waterbox also makes specific shallow frag tanks. They're really beautiful. I think
@Bruce Spiegelman got one. If you want a show frag tank, the Waterbox is probably the way to go unless you build one custom.
If there's a good deal, I may actually replace my Reefer 170 with a Waterbox 100.3.