Unlike Fins, Capital S and IG, their additional spread is dependant on the business they're doing. Often as low as zero, sometimes up to 2%. The other three offer a fixed spread increase of 0.5% (CS) and about 0.8% for the others.
Overall, if you're trading large caps the tightest spreads will be CMC, by a fair bit.
Cheers,
UTB
Edit - the above info is for fututes bets. I'm fairly sure that it's still true for Rolling. I compare the rolling price with the trades data at iii.co.uk and find that CMC's spreads are more often than not in line (sometimes better!) than the market.