Of course, depending on where you're starting from, you may have to "waste time" driving through a fair bit of England!
And I would suggest, based on prices that I've seen bandied about on this forum, that the "feeling" that the short crossing is cheaper may be correct, even when the cost of extra diesel is taken into account. Before buying the motorhome, visiting Brittany, Vendée, Charente Maritime or the Medoc, it was a no-brainer to use Brittany Ferries, because we could take advantage of the long outbound crossing (Portsmouth-Saint Malo overnight) to sleep and then take to the road refreshed with a full day ahead of us; the short crossing would have involved not only extra fuel but also driving around London and at least one night in a Chambre d'Hôte or hotel. With a motorhome, the economics may be less clear-cut. And of course, total time/distance on the road will depend also on one's starting point in the UK. Departing from the Midlands, I still prefer to use the central Channel crossings, but opt for the daytime ones (cheaper), overnighting in the vehicle check-in lanes outbound from Poole or in wild camping locations if sailing from Portsmouth. Wouldn't want too short a crossing - the crossing's part of the holiday!
Tom