Its up to you, I would prolly do both to keep from going insane. One high level and 4 low level toons will get a lot of xp for the new guys. It may not seem like it per kill, but your killing 50x as fast. When I got my priest to 60 and got set up to multi box with 4 mages at level 10. I hit Ragefire and never looked back, I did 90% of their leveling in instances boosting with the priest. I did a few quests for items wanted or were in the instance I was running at the time. This was w/o RAF and it took about 4 days played time to go from 1-60 this way. Im sure if I did it again and made some changes like only looting bosses instead of every mob, and using a better booster than a holy priest, you could cut that time prolly by 40%.