I've got the Century BOB XL. There's tons of people saying it tips over too easily. I've seen a ton of videos of people tipping it over. Tow things I pretty much always notice - they're pushing it rather than striking it correctly, and/or the base isn't filled right. Don't get me wrong, if I put the BOB XL on the highest setting and hit it in the face hard enough, it could tip over. But I don't practice punching 6'6" tall people on my bag very often.
I hit a Wavemaster XXL 5 days a week for about 6 months (it was at my local YMCA). It was filled with water, and not once did I tip that one over. I've seen a ton of people tip that on in videos and even watched a guy at the Y tip it over. Every one of them looked like they were pushing rather than striking. If you plan on running and flying kicks, then yeah, they'll all tip over. But a heavy bag would swing just as much.
I wouldn't recommend the Wavemaster XXL for other reasons though. The foam breaks down quite fast. It wasn't too bad when I started hitting it, but during that time I left clear permanent indentations on it. Of course I don't know who else was doing what else to it, but according to the staff hardly anyone hits the thing for any length of time. I was fooling around and smacking it around a few weeks ago, and it felt like a pillow. I was hitting the plastic core. Not that big a deal if you're wearing heavy gloves, but kicking the thing isn't going to feel good, especially with your shins.
The BOB is far firmer and more durable in my experience. And it doesn't have the faulty screw in mechanism that people reportedly break constantly. I really wanted the Wavemaster XXL as it's like a freestanding muay Thai bag; after the one in the Y, no way I'd buy or recommend one.
Are you sure there's no way to hang a bag? The Outslayer muay Thai bags are phenominal. I can't hang one either, so I've got the BOB XL. When I move to a space where I can hang a bag, I'll get one of thos and keep the BOB XL alongside it.
Ringside makes some great stuff. Hopefully that bag is up to their other stuff's standards.