Hello Gabe,
you can use a T1 link as an access link to MPLS service/cloud but MPLS is not tied to any specific WAN technology.
MPLS adds a little more overhead but it can be offered with higher bandwidth and it has space to grow (they can offer you 10 or 20 Mbps as a subrate of a FE or a T3 link just to say).
You cannot easily get more bandwidth with a T1 you need to add a second link and to pay for it and to manages both links.
T1 or T3 links prices for leased lines depend on distance between sites: MPLS as before ATM and FR prices are less dependent on distance because they use logical switching.
Even leased lines are carried by WAN providers inside SONET / SDH hierarchy so your T1 is carried as a payload on fiber links like STM-1 or more.
Latency can be less variable on T1 service but MPLS has built in QoS capabilities and you can get preferential treatment for VoIP packets.
You can contract SLA with the provider.
Look at what is the service availability in your region.
An MPLS service is easier to upgrade in comparison to T1 Wan service.
Hope to help
Giuseppe
