I trade CFDs with CMC and always get slippage with buy-stop orders of around 0.3 to 0.5 pts on the DOW 30. So with the 0.7 spread it's about 1 to 1.2 pts fee per trade, which is less than Prospreads 2 point spread on the YM. However, the benefit I can see with prospreads is that you are getting the actual futures prices, whereas on my CMC account the highs and lows of each hours bar has a variation of 1 or 2 points over or under the actual prices. So for example if I place a buy stop to go long 1 point above the previous hours high which was 13331 at 13332, sometimes my CMC price will make a new high above the previous hours bar whereas the futures price which prospreads uses does not. So I'm then in the trade when I shouldn't have been as there wasn't actually a new high.
So that's the benefit I can see with prospreads, as order placements and stop losses can be much more accurate, whereas with CMC I have to give a few points of space to not be stopped out when i shouldn't have been, or get in a trade that wouldn't have been filled on prospreads as the price didn't actually reach the required level in the proper market.