Ryanair uses Pisa airport as the nearest point of approach to Florence. There is a shuttle service between the airport and S. M. Novella railway station, the journey takes approx 70 minutes and it cost €7 one way.

Ryanair uses Treviso for Venice. It is 31km (19 miles) away from Venice; there is a coach service between the airport and Piazzale Roma (the nearest point to the city for road vehicles). It costs €4.40 one way

Treviso - Venice on the coach is easy and convenient and saves a fortune compared to flying into Marco Polo airport. BUT until they finish the new arrivals/departure facilties there's very little there. A snack bar and toilets each side of the passport control as really the place is a little shed at the end of the runway.
However, don't be put off it has all you need.
If getting a boarding card no. 1-60 (essential if you want to sit a family together with ryanair) then you can get an earlier coach from Piazalle Roma on the way back. You can pick up a timetable when you book the tickets for the coach on arrival. They always seem to run more coaches than advertised.

I love both and would find it hard to choose which one to go back to on a split decision....so can't really advise other than say, go for it, and choose one!

Easyjet fly to Marco Polo which is very handy for Mestre, which is where we stayed for Venice. It's on the mainland and only 1 euro on the bus to Venice, has it's own shops, bars restaurants too. We stayed in Hotel Centrale, and liked it very much.

In Florence we stayed in Holiday Inn Florence (not 'north'). It was on the other side of the arno, but again cheap and no problem on the bus, and a nice pool at the hotel. A few cafe's nearby.

It depends what time of the year you are thinking of going. Summer in both places can be extremely crowded, long queues in Venice for water transportation and in Florence for entrance to buildings, though it is easy to get to Pisa from Florence for a half day. Winter in Venice can be bone chilling. Autumn and Spring are the best time for visits if you can't take the crowds. Despite all this they are great destinations to visit and I would find it hard to choose for a first time trip.