I've got a Mission SP-1, and I like it pretty well. Right now I do well with one pedal, as my Liquid Foot lets me setup a button to change what a single pedal is controlling between 4 items on the fly. But I'll probably still add a second so I have two controls ready to go at any time. The Mission is fantastic for Wah. It works pretty well for volume too, once you tighten it down enough to stay put when you release it. I've heard some say they can't get it tight enough for that, or that it loses its grip over time. I've only had mine a couple weeks, so I can't say.
I lean towards Boss for my second pedal. I do make a lot of use of volume pedals -- usually as a master volume control at the end of my chain, as I play at church without going through the PA (due to its limitations there), so I have to watch volume carefully. The Mission works OK for that, but I think the Boss may work better. But I do like the feel of the Mission for wah. Other expression controls, I'm not sure yet. But having one of each (along with the LF's ability to use a button to switch what they do) should give me the best of both worlds. I'll be very interested to see over time if I gravitate to one over the other to the point where I'd want both pedals to be either the Boss or Mission. But having both seems appealing.