You’re only in Italy for a week. You’ll be walking around a lot, working up an appetite. Why not enjoy a serious lunch and serious dinner every day? You might want to have a picnic lunch sometimes just because there are so many great things in bakeries and delis.
The same people own Roscioli and Emma and there’s a lot of overlap in the menus. Emma is in every respect the better of the two, so I’ll never sit in Roscioli’s uncomfortable chairs again, though it’s a good deli.
If you want to eat lunch at a market, I highly recommend Testaccio. One of the best markets and one of the best less-touristy neighborhoods in Rome. While there, also visit Volpetti, one of if not the best deli in town.
Mercato Centrale is next to the Termini train station, a neighborhood I’ll go to great lengths to avoid.
Don’t waste a meal on steak if you don’t like it rare. You can get better beef in the US anyway.