The seats are fairly comfortable. They're usually in groups of four, with two facing two, and a table between them. As Nick says, the difference between 1st and 2nd class is trivial and 1st isn't worth the extra cost.
It doesn't matter which side you sit on. The views are about the same.
The site Nick mentioned lets you choose your seats from a floor plan, and shows you the discounts available on the main timetable page, but requires that you use the Italian names for the cities: Roma and Venezia.It's also no good for routes that have only regional trains on them. This site has all the trains, and let's you use the English names:
http://www.trenitalia.com/homepage_en.html
On this site, you have to choose a train and continue to the next page to see the discounts.
On both sites, use a date before May 6th, because they're not showing trains after that date, for some reason. If you want to see full information for regional trains, you have to use a date in the next week.