I had a good response typed out, but I lost it. dang

anyway, you should be doing both. Try to get in an LT interval session and an anerobic workout. one of the important things about anerobic workouts is to have incomplete recovery. If your in a race and have to push hard for a minute, your not going to have 10 minutes to chill and let your HR go back below 100. you need to improve your ability to go, then recover. so try doing intervals in the 1-3 minute range and just let your HR go back down to 65-70% of your max and then go again.

And heres a pretty good article on anerobic intervals and stuff

http://www.active.com/story.cfm?story_id=11587