I'd certainly walk to either one. In the case of Roscioli you can walk through Campo dei Fiori, in the case of Taverna Ripetta you get to see the Piazza Navona. These are two of the most famous (i.e. beautiful) piazzae in Rome. Of course, you can also walk along the Lungotevere, say, on the way back... Don't worry about the distance, on a map or Google Earth it looks longer than it really is.