Raileurope vs Trenitalia

Most of the posters on this site agree that RailEurope is a lot more expensive and has a more limited choice of trains than Trenitalia.
Trenitalia (and, in a few cases Italo or some other local companies) are the company actually running the trains in Italy. Anybody else (including RailEurope) are resellers who are liable to add a margin.
If you use a site priced in Euros, your bank will do the conversion at their rate of the day. Make sure you have warned your bank, as some may block foreign transactions unless warned in advance.

Always use the national train company to book directly. Trenitalia can save you loads of money if you book early and get the super discounted fares. RE also adds a $7.95 process fee AND delivery fee (unless you choose to print at home). They also do not show all of the trains running between various destinations. Why on earth anybody would use them is beyond me.

RailEurope is a travel agency that resells train tickets for all of Europe. There is no train called Rail Europe. Trenitaila is the national rail company of Italy that operates nearly all of the trains in Italy. You should buy your tickets and see the schedule directly from them in euro.

Italo is a new rail company in Italy operating privately but using Trenitaila tracks and stations. However, most of the stations used are the small stations and many times not near the city center as is more typical of trenitalia. Price should be about the same without special promotions..

Anybody else (including RailEurope) are resellers who are liable to add a margin.

That has been changing. There are companies such as capitainetrain.com that now have access to the reservation system of the national train companies and sell tickets at exactly the same price as if you had bought them directly. They receive a commission from the national train company. I don't think they cover Italy yet, but they do sell tickets for the national lines in France and Germany.

The tickets are the same. The only differences are those that you can identify online regarding schedule options offered, price in dollars or euros, and RE issuing fee. Rail Europe sells both Trenitalia and Italo tickets, but if you've decided which one you prefer (based on schedule, train station served, etc), then you can go direct to the source. There's no need to book online for regional (slower, unreserved trains without advance-purchase discounts); through Trenitalia, restrictions are added to online tickets and through Rail Europe those are likely the routes that have to be shipped as paper, instead of electronic tickets. Regardless of agent, advance-discount tickets (for faster, reserved trains) are non refundable and the cheapest category are non changeable.