I use both. It depends upon what I am building, my mood, how fast I want to get things done, or any one of a number of other factors that are elusive. From a practical standpoint, I don't think there is any difference in strength, assuming that both types are made accurately. For most projects, slip tenons just seem a bit easier, especially if you have your router set up to make the mortises, or better yet, you have a dedicated mortising fixture or machine.
Certainly if you're doing a through tenon, then a squared tenon looks much better than one that is rounded, at least in my opinion. But, you can still do a loose tenon that is square, you just need to square up the mortise. This really isn't very hard with a sharp chisel. In fact, it only takes 30 seconds or so.
Matt