Hi Brendan,
Thanks for the question. It really depends on whether you are getting your bets matched (at pre-race prices).
As a rough guide, we think the dual option, is for people betting with stakes in excess of £1000 per race. The idea is that if you’ve not getting regularly matched bets, you could lower your stake sizes, and bet on both Exchanges, for example.
Currently, the Betfair Starting price is beating the pre-race prices, have a look at Pre-Race Market versus Betfair SP.
Cheers
Nick