If you go in with cash you'll have more room to play with than if you take their finance. Either way you should be able to get something off the price.
Not necessarily on the 'better deal' front going in with cash.
Salesmen and dealers get a commission from selling finance packages on top of the commission for selling a car. So they are more likely to offer a better discount on a vehicle if you take out their finance package, as they'll earn an extra commission.
To get a better deal, sometimes the dealer will offer a better discount on the vehicle if you take their finance.
A savvy buyer (from people who've done it this way) is to agree the finance deal, if it stacks up as a better deal & pay the finance off within the cooling off period......IF you can do this.
Whichever way you look at it the only thing that matters is the overall cost to change your vehicle