There aren't any SB providers offering MT5, are there?
I'm getting the impression that (in the world of spread-betting anyway), the pragmatic choice is MT4 (if you want or need custom indicators or those not offered by the proprietary platform) for making your analysis & trade decisions, but use your provider's proprietary platform for execution.
(That's assuming you don't use EA's, of course).
Must admit I've never felt comfortable using MT4 for execution.