In short, please use stranded. Over the years I've upgraded my pool pad with new equipment. When I run new wire going from solid wire (more stiff) to stranded (more flexible) it makes it easier to work with.
Just saw damage done to a pump using solid wire. The plastic plate with the brass contacts where the wire's are screwed onto cracked, letting the Hot come lose and lay against the ground. Checking the screws they became stripped from trying to tighten the solid wires. Also moving the pump around doesn't help since solid wires have no give where they connect to the pump (and other devices). Symptom was the circuit breaker kept tripping and pump wouldn't work. The pump was the last thing I checked (after tearing into my suntouch controller).
Another benefit to stranded is you can (and probably should, at least with devices that vibrate like pumps) crimp on ring or spade terminals. This is an improvement over screwing the wire directly onto the wire terminals.
If you're upgrading you electrical or adding a new piece of equipment take this into consideration.