IG Markets have just lowered their margins, still not as good as CMC's 3% margins for the top 20 but their range of shortable stocks is at least comparable to CMC and I find IG alot lot better.
I use IG. They are very good. I would never use CMC as they are market makers and this is a high risk. You may think that their margin is lower, however their spread and prices will definitely go against you (IMO).