I ALWAYS wear flip flops. Although, I love wearing flip flops - they are by far my most comfortable shoes.
For me, tennis shoes are too hot - and normally I wear comfortable knit dresses to the parks, so tennis shoes just don't look right.
I also find tennis shoes very uncomfortable in the rain - experienced this only once - I was wearing tennis shoes at Epcot, then when the rain stopped, my shoes were uncomfortable the rest of the day because my feet, socks, and shoes were damp!
Flip flops can get slip-slidy in the rain, but at least your feet will dry quickly when the rain stops!
Anyway, I think it all comes down to personal preference.