Another thing that you need to factor in, is how many battery packs do you have? For example, I've got four Pro-Pulse battery (and three for my RC18MT plus a Li-Po, for a combined count of eight) packs. Since I've got so many battery packs, I charge them at 2.0 amps. I get a lot of punch from my batteries, but as BB mentioned, I get shorter running times to the pack. But it doesn't affect me because since I've got so many battery packs, I can change them out as I'm charging the next pack.
I'm assuming that you've only got one battery pack, so I'd recommend that you charge it at 1.5 amps. You should invest in more packs. You'll have tons more fun by running multiple packs!