as far as casual tourism is concerned, Thailand is probably better equipped for it and has scenery that is far more "special" than Vietnam. Vietnam is very crowded, everywhere. It's tough to relax.

If you do have a chance to visit Thailand I would say to stay away from the very popular places like Phuket and Pattaya because they are just tourist trap nightmares. Samui kind of is also. The areas of Krabi are ideal because they are wonderfully beautiful and not completely overrun with tourists. Places like Koh Lanta are just unreal. Oh Stay away from the famous Phi Phi islands too, go on a day trip to see it but don't stay there. you will be pestered by touts any time you so much as set foot outside of your resort.