Yes, just slightly.
Windows 7 is just a better operating system when it comes down to it. It has the speed of XP with the looks of Vista (but better). The little buggy things that irritated people in Vista (and the major things like bsod's) are fixed in Windows 7. They shoved Vista out there too fast, and it wasn't ready. Everybody hated it. They remedied themselves with Windows 7. It's much better!! (I've had Vista, so I know)
If you got a free upgrade, then by all means use it! I was lucky enough to get it for free from my college's MSDN AA account, but a lot of people would kill to get it!
You didn't mention the specs of your computer (RAM, processor, hard drive, etc...), but I can tell you this: 7 will use a lot less of your resources than Vista. I can't guarantee your hard drive usage will go down with an upgrade, but RAM and processor usage will go down dramatically.
If you had gotten your PC just a little later, it would have come with 7 on it already. But when the manufacturers knew that 7 was coming out, they started offering Vista with a free upgrade to 7.
It's a very simple process to upgrade. You just pop in the Upgrade advisor disk, follow the prompts, pop in the real upgrade disk, let it sit for about 2 hours, then come back and enjoy Windows 7 in all it's glory!!
I've used Windows 7 since the Release Candidate (the one that anyone could get; I recently installed the full version), and I can honestly say I will NEVER own another computer with Vista on it. My opinion of Vista was this: it worked (most of the time), it was just ugly when it did it (silly errors, "safely remove hardware" icon screwing up, green bar of death, occasional bsod, poor user interface, I could go on for days).
I've seen people like you that don't have a problem with Vista, and don't see what the point of going to 7 is. And in the case of whether or not to BUY Windows 7 separately after having the computer for a year or two, I would say no. But if it came with a free upgrade, then I say, "What do you have to lose? Go for it." If you like Vista just fine, don't go out and buy 7. However, if you like Vista and have a free upgrade to 7, you will like Windows 7 even better.
Sorry for such a long post, but I thought I needed to clear things up.
To summarize, I suggest you go ahead and do the upgrade. You'll love Windows 7 that much more.