If I had the pick I would choose the Lifehammer, but the ResQMe is nice and small and portable.
I've never needed either for myself, but as a rescuer, Ive found the Lifehammer is quite easy 1 handed.
If I was the one in the need and only had 1 hand, chances are that the belt would have tension, or I could move my body a little or push away with my hand as I cut to give a little tension. There is not that much gap on the ResQMe for the belt to fold, and the blade is on an angle so it pushes the belt edge flat against the plastic prong. (look at the pictures to see what I mean) The Life hammer has a bigger gap, but you also have the leverage of the handle to assist.

Another problem with the ResQMe is, dont let your wives/girl friends (or husbands) use theirs (or yours) as a letter opener. Little bits of paper clogs up at the blade edge so it wont cut the belt when needed, and its not as sharp after doing this for a while. (Dont ask me how I know this <img src="/images/graemlins/smile.gif" alt="" /> )