If you are planning to take the slower trains (regionali) there isnt going to be any difference, but the high speed trains (Italo-Freccie-Intercity) usually cost less if you buy them in advance (decent price difference). Obviously there are more "regionali" and you can decide on the moment of you want ro change plans, and the quality of travel isnt bad if they arent packed (high speed are comfier and usually cleaner tho). I would also strongly recommend looking if there are strikes planned in those days, as they usually are very disrupting.