Depending on your route of course but what about the tunnel?
I have crossed a couple of times with the motorcycle and wouldn't dream of using the ferry as they have not the time or the inclination to look after something as precarious as a bike. The tunnel was cheap, very fast, and I stood next to the bike holding it up for the journey. Quick and easy!
Good point, especially for bikes, but the biggest disadvantage with the Tunnel, at least as far as I'm concerned is the lack of flexibility, if you miss the time you booked, you're in trouble. That's probably not so bad on the outward journey, but it is when you come back. With P & O, although you have to book a time, in practice they just put you on the first available boat when you turn up in Calais, a big advantage if you've driven a long way. You have to specify a sailing when you book, so I pick the first cheap rate one, usually something like 21.00 hours. Driving from the Alps, traffic permitting and knowing the route, I usually arrive an hour or two before that, and I've never failed to get on the first boat, with no surcharge for being too early for the cheap rate I've booked. Reading the terms & conditions on their site, it looks to me if arriving early or late at the Tunnel terminal could be a big problem, at the very least there's a hefty surcharge for any alterations. Also, as I mentioned earlier, I have an unfair advantage of getting 50% off the standard rates on P & O.
PS And on that point about cheaper night sailings, I get the cheap rate on the outward sailing as well, leave home just after midnight, three hours or so along a deserted M3, M25 & M20, averaging about 70mph for the journey, and onto the 4.30 sailing from Dover. With the time going forward one hour, reach a French or Belgian town just in time for coffee & croissants.