I do not know much about Multitech.
Betwen Cisco and Quintum:
Quality of equipment, less troubles, less bugs to fix,more information,cisco is the best.
Quintum: Easier to setup and use, have some limitations,not perfect, but usable and cheaper than Cisco equivalent.
Requiere less training. Easier for a starter company.
If you go for Quintum,try to stay digital en the US end. Many companies now sell fractional T1 with only few channels on it.
Save yourself troubles, do not go analog on the US end if posible.
It makes for a lot cleaner disconnect signaling and the voice levels will be much better.
Get a D800 or better one of the new DX800.