Generally, plum tomatoes are best for making marinara (or gravy as it is called in Rhode Island). We used to grow Roma tomatoes for our gravy. That is until we discovered San Marzano. San Marzano originated in South America, but get their name from the area near Naples where they were first planted in Italy.
We find the taste of San Marzano to be much better than Roma and they have fewer seeds, which is a plus when you are making gravy. San Marzano are also prolific producers, so if you plant this variety be ready to share with your friends!
San Marzano is considered an heirloom. Heirlooms have two main characteristics. First, they must have been used for at least fifty years. San Marzano certainly qualifies as it was planted in Peru hundreds, and maybe thousands of years ago. Second, the seed must be able to faithfully reproduce year after year. While this may seem obvious, modern hybrid seeds will not do this. If you plant a seed taken from a hybrid plant the following year it will either not produce fruit or produce a fruit that is inedible.