I haven't tried them on the same Mac but I'd say the SSD's are a lot faster. But the Momentus XT is a lot faster than the 5400 RPM drives that ship in most Macs. The boot times on our machines with SSD are under 30 seconds and so is the boot time on my Macbook with a Momentus XT. On our SSD Macs, all apps load quickly. On my Macbook, only my most frequently used apps load quickly. That's a lot of bang for relatively little buck so I'm quite happy with my Momentus XT.
Make no mistake, though. If SSD prices drop below my "can't resist" point, I'll be swapping out the Momentus XT for a full SSD. What's my can't resist point? I'll know it when I see it. It partly has to do with the ratio of the price of the drive I'm using to the drive I'm replacing it with and the relative sizes of the drives. Since my 500GB Momentus XT is 40% full, I can't consider one of those low budget 64 or 128 GB SSD's. It also has something to do with how much I can afford to spend on stuff I don't absolutely need right now, which is close to 0 or at least a lot less than today's cost of a 250+ GB SSD.
And there's another factor. My Macbook was voted off the Moutain Lion island so I'll likely be upgrading to a new Mac rather than spending any more on my Macbook. I recently got a refurb Mini and popped in 8 GB of RAM and it's a lot faster than my MB. Maybe I'll get around to putting an SSD in that thing and let it become my main Mac.