I am no hydrodynamics engineer so take this with a few grains of salt as this is just my opinion.
Some of the fastest boats are ones that routinely plane and someone is always trying to break the last speed record. There are a bunch of factors that effect the way a hull planes: shape, weight, sail area, wave height etc etc. I'm going to say that the faster hull will be the one that looks 'fastest', meaning the hull looks like a Laser: flat bottom, gradual entry to the water and not beamy or wide. Some of the biggest ocean racing boats are like giant Lasers on steroids.
With that in mind the Buccaneer 18 looks more like a bigger version of a Laser in hull shape then a Snipe does but they are not really that different overall and they both will get up on a plane in the right conditions.
That said I'd bet money that a Snipe using a spinnaker racing a Buc 18' without one would be faster. Matched evenly I'd bet on the Buc 18' but not by much.
The 5% difference in Portsmouth ratings also make me believe that the Buc 18' could be a tad faster on a plane then a Snipe.
This website will compare 2 boats side by side: Sail Calculator Pro v3.53 - 2500+ boats
I could not find the info for the Snipe but you can enter it. They do have the specs for the Buc 18' though.