re: Italy in March or May?

Just a note, Europe's great cities are very rarely ever devoid of tourists/crowds. The Vatican, Louvre, St. Mark's, etc... pretty much whenever you go there will be lines and tourists. It's sort of like Disney, there is busy season and then really, really busy season.

However, was in Venice in late May and although a little warm, found the weather delightful. Plus, at that time of year you have daylight until almost 9 pm. Something to consider is it will be darker earlier in March.