Point of Sale Application Help

hey all...
im looking to set up a p.o.s system in my shop, it's a grocery store.
After receiving quotations from three sources, they all spoke about
database. One had Microsoft SQL, the other had MySQL and the last
person was using Microsoft Access. Now, let me tell you that, we are
planning to implement three counters, or terminals. And besides, that,
processing of data from other branches will also be conducted in the
headquarters. Now, I've read quite a lot about the pros and cons of
SQL, or MySQL. The thing is, which one should I choose?

If we are looking for a real-time application which can update from all
the stores into the HQ, and the HQ is also above another outlet, as in
the same building as one of the outlets. LAN networked and linked to a
server/advanced PC.

Do my terminals then fall under the category of multiusers?

Please help....