MPLS VPN and OSPF

Hello,

My company has bought MPLS VPN service from ISP, that connects several sites around Europe. I want to start IGP protocol inside it and tract the entire company as one AS. ISP tells me that we could use his BGP from a MPLS and tract each site as a different AS. Sites aren't big: mostly 3 persons on site, two bigger with 50 employers. What approach would be the best? Connecting each sites using GRE tunnels over MPLS and start using OSPF or BGP from ISP?

Fast convergence and faster response to failures in the network (switch to IPSec tunnel if MPLS is down) has a big factor to choose the solution.

Thanks for any tips.