What's The Difference Between Pressurized And Pressureless Tennis Balls?

What the heck is the difference between pressurized and pressureless tennis balls? And why should I care?
If you're like me, you're looking for only one kind of tennis balls - the cheap ones! I buy balls by the case at Costco so I always have a can on hand. If I happen to be in Wal-Mart, I'll buy balls there too as they are often priced at less than $2 a can (I guess whoever is setting the prices at Wal-Mart is not a big tennis player and doesn't realize they could get more than this).
But I was thinking that, since tennis balls are an absolute necessity to play the game and since I'm vaguely aware that there are different kinds of tennis balls, what are the differences? And am I playing with the right kind of balls?
Without getting too technical (or boring), here's the anatomy of a tennis ball - a rubber ball wrapped in a fuzzy fabric covering with either a solid rubber core or a hollow core filled with gas. When the core is solid, the ball is called "pressureless." When the core is hollow, it is filled with pressurized air or nitrogen so the balls are called "pressurized."
So what's the real difference between pressurized and pressureless tennis balls and why would you use one or the other?
Pressurized balls are the ones you usually find in a single can of three - probably the ones you buy most of the time. Their benefits?

More bounce - Pressurized balls feel more "lively" when they first come out of the can. The can itself is vacuum sealed to prevent the pressurized gas inside the balls from leaking.
More spin response - Because they're lighter, you can generate more spin with these balls.
More speed - Since they have less mass than pressureless balls, pressurized balls travel faster.

But these benefits are, like a good haircut, very short-lived. Within two to four weeks (or less) of opening the can, these balls will become pretty much unplayable. As the pressurized gas inside the balls escapes, these balls lose their bounce and feel "dead" or "wooden." That's why you probably open a new can of balls every time you play a match that counts for something.
Pressureless balls, on the other hand, feel a little "dead" right from the start. But, over time, as the fuzzy fabric cover on the ball wears away and the rubber inside softens, these balls actually become more bouncy. While that sounds good, the fact that these balls are heavier means that they strike your racquet with more force. And they require your arm and the rest of your body to use more force in hitting them. The result can be an increase in injury. And while the balls may become bouncier, their spin response decreases over time. A definite negative in these days when so many people, even me, are trying to generate spin on their serves and ground strokes. So where will you see pressureless balls in use? They're often used in lessons and in ball machines because of their longer life.
Conclusion - keep buying the pressurized balls in cans for use in your matches. But don't be surprised if you find yourself hitting pressureless balls in a lesson or with a ball machine. In fact, you may want to comment on this to show your pro and/or your tennis pals your vast command of tennis trivia. Or . . . not. Surprisingly, most people aren't as impressed by tennis trivia as you might think.