I would go with the HPI one. The MaH ratings are the same so I would put them on an equal playing field for run time (however, they could be totally different cells i.e. sanyo vs. noname). But nothing to make a drastic difference.
To me, the more important factor since the MaH is good and they are nimh, is the dimensions. The HPI is a hair smaller. When you go to cram one in your battery box, you will be thankful for that .02 of an inch
I agree 100% with Christian on this and especially when it comes to "craming" it in your batterybox. I run a custom 5-cell hump with Matched and Cycled Sanyo cells and it's overkill for this application but I love it.
You'll definitely get longer run times with a NImh pack.
I made my own out of 2000mAh NImh Radio Shack AA's. I had to cut the top off the battery box and find a means to keep them in, but i get a lot of run time out of them.