Ratings may make them look similar, but there's no comparison. Lipo holds voltage under load much better than NiMh, which gives you better performance. You'll see better run time from the lipo as well.

I was running 18th scales with gp1100 NiMh pacs - would get 9-12min runtime and got used to the pacs fading a little after the 1st few min run time. Switched to lipo, slightly bigger at 1300, but now get that fresh charged NiMh type performanace nearly all thru the pac. Huge difference in run time - the lipo gets 20+min routinely, have seen as much as 25min.