I would advise on full service for a couple of reasons:
1) Lifetime upgrade. I you EVER want to upgrade, you don't have to worry about the hassle, not to mention the loss you'll incur, with selling a diamond privately.
2) You can be 1000% sure of exactly what you're paying for.
3) You can brag to all your friends on how superior your diamond is just by providing a link to the website! (not that anyone would brag, but it'd be nice).