Ricky_Bobby, first, awesome handle and welcome to the board!
I've had my box in my shop now for a few weeks. I am really happy with it. I looked very hard at the USG S3 option... that gray is pretty sweet, along with the blue and the green. I think they're good boxes overall and of course there's thousands of them out there in the world. My old box that this replaced was a first generation USG 44", which was kind of the holy grail of budget tool boxes here back in the day. In fact, my search for a tool box is what led me to the Garage Journal in the first place. I ended up selling it for basically what I paid for it. That's just crazy. I had exactly zero issues with that box during its life and when I sold it, it was in very good shape. My shop stays clean but cluttered, so neither dust nor moisture are things that I worry about too much
When I saw the new S3 boxes drop at SEMA a while ago and saw the gray, I had every intention of getting one.
But... I saw the Masterforce box in a Youtube video and was very intrigued. I don't have a local Menards and was eventually able to see one during a trip. As I said in a post above, the drawer layout and overall feel is what sold me. I really like the multiple shallow wide drawers--three on the MF and two on the USGS3. That's a very personal thing so you gotta go with what you want. I also love the drawer latches. Last but not least is the drawer depth. 24" drawers are awesome. This amounts to almost 9000 cubic inches of extra storage over the USG box. This last part sealed the deal for me.
As for the open bottom, I never really noticed it or thought much about it to be honest. Other than snow on vehicles during the winter, my shop floor stays pretty dry. I won't worry about it too much. I think you'd be happy with either box in the end.