Specifically Siesta Key area.

March-April...OK, do you want more or less guaranteed warm weather (as much as we can guarantee anything here in Florida, that is). Are you wanting lots of beach time?

And as to Sanibel (and perhaps even better, nearby Captiva) you can wade through the "this one is better than that one" stuff FOREVER. We all have our favorites but you need to be practical, weather-wise because Florida encompasses several different "weather usuals" during that time of year.