Rock Lobster is more 'XC', and designed for a 100mmish fork (I run Rebas at 115mm on mine; no problems).
Frame is not as 'long' as an On-One; my 15" frame has an effective top tube of 22 1/4", I think the On-one in a corresponding size is 1/2" longer?
I can't comment on the On-One, but I love my Lobster. I think the geometry is spot on, for me, and it rides really well. I was very impressed actually, as I din't expect a 'cheapish' frame to be this good. The details like the ring-reinforced headtube, Ritchey dropouts and forward facing seat clamp are all really nice touches. The welds seem pretty good, and it is a nicely made frame.
I'm light, at 9 1/2stone, and find the frame very comfy. It's made of the lighter end of the 853 tubing, as far as I am aware, so if you're a bit of a biffer, or want something a bit more 'hardcore', then maybe not the ideal frame.
At £210, it's a no-brainer, if you think it's the right frame for you. That's a ridiculous bargain. I'd buy it in a flash; mine cost £400 inc a Reba Team fork; I just coon't not have it!
Fantastic bike, I love it: