It’s hard to come up with an analogy that everyone can appreciate and which fits well enough that it will help, but I’ll try. Consider the first time you saw a TV show or movie in HD – perhaps you bought an HDTV, perhaps you upgraded from digital cable to HD, or you bought a Blu-Ray or HD-DVD player. Maybe you just saw one at Best Buy playing your favourite summer blockbuster. You see what’s on the screen, and it’s still the same, really. It’s the same movie you’ve enjoyed in the past, the same TV shows you’ve always loved. The plot hasn’t changed, the writing isn’t any better. It’s quantitatively clearer, but that provides a qualitative improvement.

If you’re a gamer, the comparison is easy. Do you remember the first time you upgraded your video card to the latest and greatest? The frame rate increased, you get more texture quality, it’s smoother. There’s less jitter, scrolling and panning is smoother. You didn’t really notice it before, but now it feels better. It’s more natural, it flows better.

And that’s really the key. The iPhone isn’t faster, because ‘faster’ doesn’t connote the right differences. The iPhone is more fluid. Moving from one application to another, or from one screen to the next, or scrolling around on a website or reading your e-mail, it flows better. It’s not just that the current iPhone is slow, it’s more that it breaks your stride.

Here’s another example. Let’s say you’ve got a room in your house with doors on two sides, directly opposite. On the other two sides, you have a couch and a TV, and in the middle, there’s a coffee table. Let’s also say you have to walk through this room to get from your bedroom to the kitchen, so it’s a trip you make fairly often. If you put your coffee table directly between the two doors, then every time you cross this room, you’ll have to go around it. It’s not a big deal, but it throws off your momentum. You’ll have to step to the side twice every time you go through the room. You’ll also have to go through the line of sight from the couch to the TV, so you might have to slow down if someone’s playing games or watching a movie, waiting for the perfect opportunity, and you’re more likely to bang your knee while going around it, maybe if you’re in a hurry or it’s dark.

Using the iPhone 3G now is a lot like that. There are a lot of situations now where the flow of action from one activity to the next is disrupted. The straight path from point A to point B has a detour in the middle, an obstacle, a delay, and it slows your momentum ever so slightly. Enough of these delays and it becomes subconsciously frustrating. The iPhone 3GS does away with that.

Now here’s the key, and here’s why it’s hard to quantify. You probably didn’t notice any of those delays. You go around that coffee table so often that you don’t even realize it’s there, but on a subconscious level, it bothers you. You don’t think to move the coffee table because you don’t realize it bothers you, but if you moved it a half a foot out of the way, you’d decrease your subconscious frustration ever so slightly, and you’d find yourself enjoying your home more (and you’d have fewer bruised shins to show for it).

I didn’t dislike Twitterrific on the iPhone, not really. It was ok, but not great. The UI was nice, but I just didn’t really care for it, preferring Tweetie. I couldn’t tell you why, I just didn’t like Twitterrific’s interface, or maybe it was the colours, or maybe it was just kind of convoluted to use. I don’t know why I didn’t care for it, but I didn’t.

Now, however, I know. Twitterrific wasn’t slow, you have to understand. I wasn’t sitting there, staring at loading screens or progress bars, waiting for it to accomplish the task I’d set it to. The real reason I didn’t care for Twitterrific was because there was some subtle, imperceptible lag when moving from one action to the next. I didn’t notice it consciously, but subconsciously it bothered me, it made me dislike the app for reasons I didn’t realize or understand. On the 3GS this week, I used Twitterrific exclusively. I didn’t try any other Twitter apps, and it didn’t occur to me to want to. Twitterrific was great, and I was happy to keep using it, because that unconscious frustration with minute waits and imperceptible lag was gone.

And that’s what it feels like to move from an iPhone 3G to a 3GS. It’s a more satisfying experience, a more enjoyable device, but not for reasons that you might think. It takes better pictures, but I don’t really care. It records video, but I don’t really care. It has faster data access, but I don’t really care. It has voice control, but I don’t really care.

The real difference between the iPhone 3G and 3GS isn’t something that you’re likely to notice unless you’re looking for it. You’ll enjoy the new phone for reasons that you can’t really explain, and when people ask why you paid an extra few hundred dollars for a not-so-different phone, you won’t regret it, but you won’t be able to defend your decision either, because you can’t reduce the experience to numbers.

It just feels better.