Do you want to use web direct?
There are no hosting problems and the file type is the same (*.fmp12). The difference is that there are some FMP 15 features that will not work on FMS 14. You might check out the new features that came out in FM 15 at FileMaker Pro 15 Help In general most things will work easily, but you just have to test it out and review the new features that would be missing.
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In a lot of cases there is some negotiating going on between FMP and FMS about who will do something, in a mixed environment this may be suboptimal and could be slower (making the client do more work) than say an all-FM15 deployment.
Shouldn't be too big an issue with 14+15 but it is something to consider.