A lot of your decision comes down to what you're going to be doing with the box and when you expect your next jump from OC3 to OC12(or greater). Also, you need to consider your comfort level with JUNOS vs IOS. If you're cool with JUNOS then multiple M series boxes are worth investigating. Our experience with them has been almost nothing but positive, plus they will allow you to expand to greater than OC3, providing you with some future proofing.
One additional data point is that due to excess forwarding resources its possible in JUNOS 6.x to condense many physical 760x and 750x down in to 1 or 2 M series using ithe virtual router capability. In a nutshell each virtual router is its own administrative domain so you can have different staff maintain different virtual routers on the same physical box. Fewer physical boxes might be an advantage for some. See release notes for JUNOS 6.x in the tech pubs area of the juniper.net web site for more details on virtual router features. -Rob