I prefer using canned tomatoes over fresh because it gives the salsa flavor that is closest to one served at restaurants. While fresh is great for Pico de Gallo, if you want that Mexican style restaurant flavor, use canned tomatoes. However, if you really want to use fresh tomatoes, it's possible! You can use fresh tomatoes and simmer the salsa with a little cooking oil for about 15 minutes to get similar flavor. Using just fresh tomatoes, won't give you this exact flavor though. If you want to use fresh, use about 1.25-1.5 lbs. of fresh Roma or San Marzano tomatoes for best results.