Best train to take from Rome to Florence? Fast or seeing the countryside?

It really doesn't make much difference. You trade speed for price. The slower trains are cheaper and we are never in a hurry. What is it worth to you to save an hour travel time?

Is it just an hour difference? which train is the most comfortable? I'm not in a hurry and would like the best experience. What about the trip to Venice from Florence?

The answer is still the same. You can check times and schedules at trenitalian.com

There are 50+ train runs each day from Rome to Florence. Most are no-stop high-speeds that whisk you there in 1hr35min. The fare is 45E. There are two other train types that make this run without changing trains. The IC train costs 30E and takes 3.0hrs. The Regionale Veloce costs 17.10E and can take 4.0hrs. To get the less expensive trains, you will have to check out the train schedule as they don't run as frequently as the high-speeds. It's all your choice. Here's a sample link to the schedule. http://orario.trenitalia.com/b2c/nppPriceTravelSolutions.do?car=0&stazin=rome&stazout=florence&datag=30&datam=03&dataa=2011&timsh=08&stazin_r=Staz_DA&stazout_r=Staz_A&timsm=33&timsm_r=33&lang=en&nreq=5&channel=tcom&npag=1&lang_r=en&nreq_r=5&channel_r=tcom&npag_r=1&x=47&y=10

The scenery will be the same. But on the slower trains you'll get to see lots of train station platforms. The high speed trains will be more comfortable and luxurious. Unless cost is a major issue with you, take the high-speed train. Your time spent seeing sights or sitting at cafes is typically more valuable than the extra cost.

Thanks everyone. I think I'll opt for the faster trains...but I noticed the information isn't out yet because my trip is in September.

There is rarely any difference from one quarter to another. Maybe a few minutes here and there but no major changes so if you want the schedule for September just same day of the week for April or May.