When to harvest tomatoes

As I have mentioned in some other posts, my neighbor has been kind enough to give me 3 heirloom tomato plants. All of them have several nice green tomatoes on them. The biggest plant, however, also contains a very large and nearly red tomato. Do I pick it now, or let it hang on the plant until it is all red? I have never grown tomato plants before...any advice would be appreciated!

Maria,

Since there's plenty of time left to summer, let it hang until ripe.

In my view, you should pick tomatoes before they ripen under two circumstances: the season has run out and there still are unripened ones out there; it was a rainy, crummy season and you want a tomato before it bursts.
Green tomatoes will ripen on a counter with sun exposure and in a paper bag, but they never are as good as fresh off the vine.

Depends on what I want to use them for. If I am going to cook them I wait till they are dead ripe and soft. If not cooked I prefer the higher acid tang from an under ripe, yellowish-red firm tomato.