Stock: Canned, Bouillon Cubes and Powder

Commercial stock or broth products come in several categories. Of the ones you ask about:

Canned

Bulky, but ready to use. May be the highest quality product of the three you asked about, with the best flavor. Especially low salt versions contain less salt than the cubed or powdered alternatives.

Cubes

Contain lots of salt. Must be dissolved in hot water before use. May not be as high a quality product as canned. Requires little shelf space. Often inexpensive on a per serving basis.

Powder

Essentially the same as cubes, but not yet pressed into cube form. Somewhat easier to dissolve than the cubes.

Concentrate

While you did not ask, concentrated stocks are also available on the market, which are almost like a jelly that you can add to thicken and enrich sauces, or reconstitute with water to use as a soup base. These tend to be marketed at commercial kitchens, and may be a higher quality product than some of the others.

It's hard to generalize them since within each category there are vastly different qualities.

Read the ingredients list carefully.

• Bouillon: With some exceptions (buy these ones), you'll find most cubes contain little if any meat based stock, and are mostly salt, yeast extract, and dehydrated vegetables plus a load of MSG. The best ones are the ones that are low in sodium and contain little fillers. They also tend to be more expensive. The good ones may allow you to provide a lift to the dish but not base a good dish on them.

• Powder: Here you'll mostly find corn starch, MSG, salt and some powdered vegetables plus thickening agents. The chicken ones may have some real chicken broth based powder, and it's very hard to actually find beef stock in powder form. While many restaurants use this type, the professional chefs avoid them and typically shake their head at dishes made using these.

• Canned: you might get some honest stock in this form. Again, read ingredients and check for salt.

• frozen stock: you can typically find honest beef and chicken stock in frozen tub containers in the freezer section. They won't be as cheap as buying some soup bones or making your own.

• Tetrapack: In North America, you can find Pacific, and Imagine brand of broth in tetrapack form. They (and a handful of others) do offer low sodium organic broth which tend to produce the best result from any store bought product and contain little fillers. The 'major' brands also have organic lines but when you read the ingredients you'll know how little actual stock you're getting.

I have a favorite option, it's a concentrate or "base", like SAJ14SAJ mentioned. Costco now carries it, making it also an inexpensive option - Better than Bouillon, the organic, reduced sodium variety. I almost always make my own broth, this product is great for stretching homemade or enriching it. In a pinch, it's even OK just mixed with water. It was included in America's Test Kitchen tasting test of canned broths, it tied (or almost tied) Swanson's canned broth for 1st place. Other answers have very well covered the pros and cons of each type of chicken broth product, this one just happens to be my favorite of all of them. All pros, no cons.