I have been experimenting with iRunner for the past few weeks. It's much more sophisticated than RunKeeper for heart rate monitoring (specifically zone tracking); beyond heart rate there were a few other things I felt were better although RunKeeper is still sexier in a Apple-like way (simple, smart, base functionality done very well).
If anyone's interested, I put a longer version of my thoughts on my blog:
p.s. just my thoughts, feel free to debate my views as much as you like :^)