Like Kaz says the tunnel is quicker but costs a tad more... I've done the trip across for a few times now and driven to Belgium/holland/germany... And the crossing being that much quicker helps. Depends though on your time schedules too, if you have the luxury of taking your time or if like me have mostly gone for weekends/long weekends.
Have already Planned on driving to Spa for this years F1 race away for four days got the euro tunnel for £58 return.
James