I would say tile. But that's not why I'm chiming in.
The edge has to be perfectly level, or you won't get an even sheet of water. It might flow more in the middle, or on one side or the other. Even to the extent of leaving one area dry. If it's not installed with care and skill, PBs have been known to hide that fact by turning up the pump RPMs. That raises the level of water in the spa, and disguises the fact that the spillover isn't level. Which works great for them, as they drive away, but leaves you with two problems: you must always run high RPMs to get a decent looking spillover, which costs you money, and you can't dial the effect way down, for a gentle trickle sound effect, which is nice to have at times.
Point being: raise this issue with your PB, so he is clear about your expectations, and check the level yourself, before they set the tile, and directly after they do. Don't pay them until you're satisfied with the spillover effect. If you have a variable speed pump, dial the RPMs way down and see how the spillover works at "low volume." Even if you don't have a VS pump, you might someday, and you want this spillover to work well then. If you don't have a VS pump, you can still check the spill over by running a hose into your spa. If your spillover is an even sheet of water at the volume of water a hose can produce, that would be an excellent job, though, admittedly, not something reasonable to expect out of a tile surface. But you get the idea. Check it now, before he's paid and gone.
Welcome to TFP! Come for the spillover advice, stay for the water maintenance advice. If you're interested in taking care of your pool in the best, most cost effective way possible, stick around. You'll be glad you did!
We are adding a 8x8 rectangular spa to existing pool. Spillover is 6 ft across. Spillover height is 2 inches. The spa is raised 12 inches above the pool. Leger-stone around the spa.
