I don't understand the technicalities of Access Vs SQL but I can share this with you:
I was running on Access, was having a lot of trouble with my Web Hosts, had to clean up the Database regularly by deleting posts, Forum was running very slowly, sometimes almost grinding to a standstill etc etc.
I now host with WebWiz (which may be a big difference as well) but we are running SQL. It is brilliant - Has been like this since early February and not one problem (touch wood)
I know Bruce & WebWiz can't recommend SQL over Access strongly enough.
Our users have posted 109,302 Posts in 6,986 Topics in 10 Forum(s)Regularly have 200-300 Users online.
Hope this helps