One point to note is that, although you are able to sustain longer by having the Center of Gravity (CG) directly above your hand in the case of the Merlin and Blackbird, for the same reason, the inevitable up-down bumping motions when you walk will also easily translate to vertical bumping of your camera too.
The offset handle of the Glidecams is very accommodating in this aspect. But you better have a strong wrist.
I've got both Merlin and Glidecam and tried the Blackbird too. I can run with the Glidecam and produce stable shots, but I can never run with the Merlin or Blackbird (the video will have very visible vertical bumps thru out, unless your hand maintains constant height all the time).
However, I'll still recommend the Merlin if you are trying out a stabilizer for the 1st time. You'll enjoy it more since you'll be able to "fly" with it for longer durations. Glidecams will turn you off right away when you can't withstand the pain in your arm. Have fun.