People are going to use the seat in the spa as a step - going in and getting out. Tile is slippery, I would not use it for the seat in the spa. You can use a bullnose tile on the edge of the pool steps which help people see where the step ends. I would stick with one color pebble tec for both pool and spa and all the waterline tile should match, including the overflow rim from spa to pool. When selecting tile keep in mind that it will need "cleaning". Calcium deposits will form and they have to be removed, by hand. This is more difficult and time consuming if you choose a small tile. If you have the budget (mucho $) a perimeter drain is the best (no tile). Perimeter drains require an installer with years of expertise in building this style of pool.