I prefer a cotton for the summer, as it can act as a natural wicking material. However, some synthetics will work a lot better. As for the SS vs LS, long sleeves would prevent sun burn on the arms, while short sleeve might be more comfortable. In addition (and I am sure with landscaping it is not a huge problem) the High Vis Green or Orange would be best as brighter colors will absorb less heat than darker clothes. Just some thoughts.
In the Army I work BDUs in 80-90s with close to 90% humidity. I thought it was a great material and had a cotton Tee and it was great. No sun burns. Darker colors though did make it a little hotter but I lived.