For the most part, cereal grasses are much more similar than they are different.  That said, wheatgrass and barley grass will perform the same function.  The biggest difference is that barley grass is very bitter and wheatgrass is not.  Since cereal grasses are essentially identical, we might as well eat the one that is most agreeable on our palette (wheatgrass).

The basic philosophy behind wheatgrass juice is that most of us have been eating a great deal of food that due to processing, is missing critical nutrition.  As a result, your body can literally run out of fuel and this causes a variety of problems depending on your specific genetic susceptibility.  What we want to do, is get great fuel into your body.  When you do this, your body will assimilate, extract and apply nutrition as needed.  For this reason, wheatgrass juice shouldn't be regarded as a treatment for a specific malady, but rather as fuel that powers your entire body.

Here is what makes wheatgrass juice terrific.  When taken in a living format (fresh, frozen or less so freeze dried), it has a small amount of a large number of nutrients.  Because it is already juiced, it is essentially pre-digested so it is extremely easy for your body to assimilate the nutrition. 

What can you expect?  Everyone responds differently but people often report better sleep and more energy.  They also report that health concerns and chronic problems are lessened or disappear.