For a start forget about the DMA/MM nonsense. Secondly, go along to the seminars, they will be sales focused but that doesn't matter. Get them to go through the trading platform either at the seminar or in a one to one situation to make sure the platform is any good. Even better if they give you trial access.
I've used CMC and have found them good for trading aussie cfds and FX. I'll have a go at doing the indices when I get round to it.
Just make sure you're comfortable with using the software as that is where you will probably place most of your trades - that's the main reason I went with CMC was because I could go into their office to learn the software.
