I like a leather strap on just about any watch in the winter, but I do agree that leather and hot weather do not go well together. Where I live it is just starting to get warm & I am still wearing a leather strap for today, but it won't be long before it is strictly rubber or metal. The other reason for metal or rubber in the summer is for practicality. What do you do with your diver with leather on it when you want to jump in the lake/pool?
Best options for this one for this time of the year are probably rep metal or rep rubber. If you are willing to splurge I am sure that the Gen Rubber is good too. I don't know how good it is, but Zealande made a Rubber strap specifically for this 2018 Omega smp 300 here:
https://www.zealande.com/en/rubber-...n_additional_tang_buckle-no_additional_buckle
The Zealande strap isn't cheap @ $175, but still about 1/2 of gen. I think I will probably give it a try and then when I decide that it isn't worth it I will sell it to someone for a bit of a loss. I might even do the same with the Gen, but I am having a hard time coming to grips with a $300 piece of rubber