I've used both.
My personal experience is that I slightly prefer mei tai for front carries, but prefer Ergo by a mile for back carries.
I could just never master getting him on my back, straps wrapped and tied correctly, etc in a parking lot w/my Babyhawk. When I did manage to get him on my back, it was never comfortable for more than 10 minutes or so.
Pros: I find the Ergo to be super easy to adjust, super easy to get baby on my back and most importantly, super comfy for long periods of time. DS2 is 8 months, 20lbs and loves being in the Ergo on my back. He falls asleep often and the sleeping hood on the Ergo comes in handy for that.
The only con I have about the Ergo is how it gives me an, um, "muffin front."
To get a good tight fit, it totally makes my belly fat stick out around the waist strap. And I'm a pretty petite girl, 5'1" 115lbs. But I have had 3 kids, so there is NO hiding that w/the Ergo. At least w/a mei tai I could hide the belly w/a big knot or bow.