Your glass tiles are 1x2s, right? I'm not sure exactly where you mean 'on top' of the spa. The top edge?
Our glass tiles are 1.5" x2". They had no trouble installing that size on the top edge of the spillway, the slope of the spillway, the circular benches in the spa.
Look at the pictures of my spa with the tile in my pool build. See if there are any similarities in our surfaces covered with glass. In some places, the tile guy had to lay the pieces one at a time rather than using the sheets of tiles. That may be what your PB is trying to avoid. If so, I think the 1x1s would look just as great.
I'm not sure I understand where the area is you mean, so I'm having to guess. But when you say on top of the spa, I can't imagine a plaster finish on top. I would want the glass tile.
Let me know if I'm confused or confusing!!
Suz.