Well I am looking to move host due to downtime with my current host. My forum (sig) gets about 150 200 unique hits a day but am looking for a good reliable host.
I have used both and am currently hosting a site with site5. There are both incredibly reliable and support friendly hosts with knowledgeable server admins and great uptime statistics.
What kind of forum software do you run? This is not a part of the reliablity issues but I'm curious. Otherwise it is important how much do you willing to pay? I think that reliable hosting cost definitely more than $5 per month, especially when you need more real space and monthly transfer quotas, good support and a web host which would help you to develop your project.