The price difference, given that a enhancer or kilo of DME will be around £6, isn't that great. But the reviews for the 2-can kits always mention the better flavour compared with the 1-cans.
Of course, if you are brewing a 1-can kit with brewers sugar (£3) or white sugar (£1) then the two can is a lot more expensive - but it will also mean the 2-can kits will taste that much better by comparison.