People have different opinions, but I'd have to say the freshest squeezed is the bestest.
Mine is something like twin gear Green Machine so it can do the parsley and wheatgrass, which the grater/spinner type can't do. It's not exactly fun or easy to clean, but then I don't care for the kitchen work. I like research! It might take 5-10 minutes to brush all the pulp out of the various pieces.
You can do a whole day's worth or even 2 day's worth. The enemy of the fresh juices is oxygen. Use glass jars with lids and fill them to the very top. Some people even freeze various juices in ice cube trays and stuff like that. Even frozen it must be a zilllion times better than packaged fast food!!!
Oh and the alkalizing effects, yes many say that acid tissues are a host to diseases. As we age the acid builds up, and even robs calcium from the bones to buffer the acid in the blood, because the blood must stay in a very tight pH range. The green leafies are great alkalizers, and one of the best is wheat grass. Charts are all over the internet. I was told Kale is very healthy to juice.
I think Weston A Price was the most brilliant as far as nutrition.
http://westonaprice.org
He studied healthy groups of people in the 1930's and found a vast range of dietary practices. The key was eating from the land they lived on and nearby waters. The Swiss had mostly milk soured into yogurt and they highly revered the cream. The Masi (sp?) in Africa ate no vegetation at all. The most balanced and robust was a Gaelic people who ate mainly fish and oats. The real evil when these people went off their native diets was the "foods of civilization" - the canned, processed, refined and devitalized "foods of commerce."
Fermented foods are another category sorely missing from most diets today. The refrigerator was one of the worst inventions health wise. Before the refrigerator fermentation was one of the main methods of preservation of food.