Just look at it as a total cost per trade. It will vary by individual for instrument traded and typical gain, but it's just spreadbet spread minus market spread (in financial terms) versus DMA commission plus expectancy x 18%. Leverage costs may be relevant for those holding overnight as spreadbet interest charges tend to be relatively expensive.
The other difficulty though is comparing quality of fills on entries, exits and stops. If some of the spreadbet threads are to be believed there are some pretty sharp practices going on that cost spreadbetters money, but how do you quantify that?
Remember also that the first £10k (in round numbers) are 'tax free' under DMA as they will be inside your annual allowance. I wonder how many spreadbetters attracted by 'tax free' actually make more than £10k pa.
How ‘bout an example:
Let’s say I trade FTSE futures / spreadbet. Comms on the futures are £3.40 R/T per contract and effectively £5 for the spreadbet (ie 1pt spread which is 0.5pt spread over the market).
I expect to place 800 trades a year (averaging 4 trades a day over 200 trading days) and trade 1 contract a time (or £10/point). I have a 70% success rate with, on average, a 10 point target and 5 point stop loss.
Spreadbetting my profits are (70%*100+30%*-50)*800-(5*800) = 40,000.
DMA my profits are (70%*100+30%*-50)*800-(3.4*800) = 41,280. Less 18% tax after my annual allowance = £35,668 In this example I would be better off spreadbetting, BUT not by a huge amount and this is all highly dependent on my ability to get the same fills as I would on DMA entries, exits and stops. Obviously you can change the parameters to change the outcome - eg if you were betting the FTSE with a two point spread the s/b profits would reduce to £32,000 and you'd be better off DMA even with tax.