Personally, I would use fiber. Depending on the hardware they are providing, it may not work by default with unsupported SFP+ modules. I know some of the legacy Cisco equipment that I have used would put the port in an error disabled state if you used a non-cisco SFP. There were ways to work around it, but I always try to avoid situations like that, especially when dealing with equipment that will be managed by another provider. It helps avoid the "It's fine on our end, the problem must be the unsupported SFP+ connection you supplied" issue if there are problems in the future.