I've had wind stability issues with my Megamid at times - it's all about how well the stakes hold. If you have good ground and good stakes, they can stand up to pretty strong winds. But if the ground is soggy or soft and you have skinny little stakes, they will pull out and down it comes. This actually has nothing to do with the fact there is no floor, it is due to the fact that you've got center poles and are relying on the stakes to hold the tension that keeps the shelter up. This is more of an issue with the Megamid or Megalight than the Beta or Twin peaks, since the latter two are smaller and so present less area for the wind to exert force on. In other words, pyramid style shelters work fine in the wind IF you choose your spots carefully and carry good stakes.
And by the way, as far as I can tell (having done some research before buying a twin peaks) the twin and beta are very close to the same size. The areas quoted are not the total covered area in either case - in the case of the Beta it is the area of the Beta floor, which they consider the useable area. I read about one person using the Beta Bug under a twin peaks, it worked fine, so they have to be pretty close to the same size. The Twin has slightly heavier fabric, and has a vent at the rear peak. I also like the way the door opens up one side of the front instead of up the middle.