The All American metal to metal seal is by far the best pressure canner. I can between 600-800 quarts a year (and more as our family grows
). A rubber seal needs to be replaced every two years at that rate of use.
The All American also has automatic pressure control. This is a huge safety and time saving feature. Of course you don't want to turn your heat high and go to town or anything, but you don't have to watch it as closely because it has the adjustable metal weight instead of a just a single weight which needs to be controlled manually by frequent heat adjustments.
The All American has both the adjustable weight and a pressure gauge.
As far as size... choose according to your needs. I don't do many two quart jars, but it is nice to have one tall enough for that. The size that holds 7 quarts fits nicely on a standard stove element, can do several two quart jars, or (11 I think) pints at a time by double stacking. Lots of my recipes and methods of gardening are gauged to the 7 quart capacity. You can run fewer than 7 quarts if you need to.
Here are some pictures and more info.www.homesteadhelpers.com/get_item_aapc_all-american-pressure-canners.htm
All that said it is possible to pressure can with a cheap Presto canner from Walmart
My mom did over 1000 quarts a year for 18 years and I borrowed one for the first year I was married, but when it comes down to really getting into home canning it is well worth the $ 200- $ 250 for the All American. It will last a lifetime!