© Denzil Green
Tinned tomatoes can save you a lot of work, are usually cheaper than buying fresh tomatoes (except for those few crazy weeks in late summer when green grocers are practically paying you to take tomatoes off their hands), and are often far more flavourful than the tomatoes we get for the rest of the year (read: 98% of the year.) The reason is that because they are going to be canned, they can be allowed to ripen longer. Tomatoes destined to be canned as chopped tomatoes are allowed to ripen the most.
Consider always keeping tinned tomatoes on hand in large quantities. You can get them tinned whole, or tinned chopped. For most general cooking, it really doesn't matter which you get, though you can always buy some of each. When they're on sale next, consider doing a big stock-up, if you have the room.
For making large batches of spaghetti sauce or chili to freeze, you can get the huge "feed the Eastern European refugee trains" tins from big warehouses such as Costco/Price Club.
The only complication with tinned tomatoes is when recipes call for "1 can of tomatoes." You might have no idea whether they mean a small tin or a large tin. You just have to wing it like the rest of us. Either guess ahead of time by weighing up in your mind what the proportions of the other ingredients are, or open the large tin, use as much as seems right to you, and toss the rest in a freezer bag and freeze.
If recipes call for a can of tomatoes, drained, drain the tomatoes into a bowl through a colander, and then add the juice to a small tomato juice bag or tub in the freezer for making various sauces with in the future. You could drink it, too, if you like tomato juice.
Most Canned Tomatoes used to come from Italy, in particular the brand named "San Marzano", until in 1989  the United States slapped a 100% tariff on Canned Tomatoes from Italy and Spain in retaliation for other EU measures. This had the side-benefit of encouraging American consumers to buy Canned Tomatoes from California.  If the imported tomatoes are canned in purée instead of sauce, the duty is lower, only 12.5%, because then they qualify as sauce products.
Cook's Illustrated studied Canned Tomatoes in 2005. They found:
- tasters' preferences were not affected by variety of tomato in the can, but they were affected by the liquid the tomatoes were packed in. Tomatoes in juice were rated as tasting fresher; tomatoes packed in tomato puree were rated as having a stale or overcooked flavour to them;
- tasters' preferences were affected by the addition of citric acid, which most brands add to ensure flavour consistency throughout the packing year. Those brands without added citric acid were rated lower in taste and freshness;
- canned tomatoes with added calcium chloride to firm them up were preferred in cooking, as it stopped them quickly turning into mush;
- drained weight contents of 28 oz cans of tomatoes varied from 8.8 to 12.8 oz.
Their recommendation was to look for citric acid, calcium chloride, and packed in tomato juice on the label.
A No. 2 1/2 can of tomatoes = 27 to 29 oz = 800 to 850 ml
A No. 300 can of tomatoes is 14 to16 oz = 415 to 475 ml
A No. 303 can of tomatoes is 16 to 17 oz = 475 ml to 500 ml
16 oz (475ml) can of tomatoes = 2 cups tomato and juice = 1 cup drained tomatoes
28 oz (825 ml) can of tomatoes = 3 cups tomato and juice = 2 cups tomato, drained
35 oz (1 litre) can = 4 cups undrained = 2 1/2 to 3 cups drained tomatoes.
Some say canned tomatoes are good for two years; others say three years.