Should I use Sunny Day or Rain Dance?

So, I'm playing pokemon tower defense and I'm having trouble with the Blastoise on Viridian City. I have a Magneton with Thunder and an Exeggutor with Solar Beam. Both need substantial leveling. I'm not sure which single pokemon (I'm only intending to put a single pokemon on anti-water-coverage) will be best able to tackle the challenge. I feel like the rest of my team can be tweaked to get benefits either from sunny day (Heat Wave, Flamethrower, lots of solar beam spam) or Rain Dance (Thunder, Aqua Tail, Brine, Hydropump). I don't know how much the cooldown reduction is for Solar Beam so I don't know if it ends up with higher stats than Thunder or not. Assuming it doesn't, I don't know if the difference is enough to make up for Magneton's lower special attack. Which is better, and by how much (assuming equal level)?

Gary's Blastoise's main strength is it's high Def and Special Defense, further boosted by the fact it uses Iron Defense as soon as it enters the field. I still don't know whether Rain Dance+Thunder or Sunny Day+Solar Beam is better, but Magneton can learn Metal Sound which, when coupled with Thunder+Rainy Day, deals enough damage to the Blastoise to take it down (Exeggutor doesn't seem to have access to any similar move). Rainy Day also lowers the power of Arcanine's Fire Fang, if he gets through the first time or you take too long to defeat him, while Sunny Day would boost it.