This topic is oft debated in many Mac forums, and does not really have a concrete answer; a lot depends on what you do with your Mac, and what Mac you use. Before deciding whether to shut down or sleep your Mac, you should know (a) what happens in each process, and (b) the pros and cons to both practices.
Many computer users (myself included) almost NEVER shut down their computers. There are many positive benefits to this (especially if you're a Mac user). When a Mac goes to sleep, almost every component inside the computer is turned off; what is not turned off is set in a 'low-power mode'. The contents of the RAM and Video RAM are saved, the network ports might still get a little power, and a few other subsystems stay on, but pretty much everything else is off.
On a laptop, this mode uses VERY little battery power - if you're going to use your laptop again within a day or so without the power adaptor plugged in, this could actually save battery power, as shutting down/starting up uses a lot more energy (much more disk access, processor power, etc.). On a desktop, this could save a little energy (thus a few $$$), especially if you put it to sleep a few times a day when you know you won't be using it.
The best advantage to sleeping is that the computer (a) goes to sleep almost immediately, allowing quick transport of a laptop, and no vigilance after clicking 'Shut Down...' on a desktop, and (b) wakes up almost immediately, allowing you to get to work right away.
Shutting down a computer has a few benefits as well. One is that NO power is being used at all - for laptops, it's best to shut it down if you're not going to use it for a day or two, as it will save a little bit of energy and heat from being wasted in your laptop. On a desktop, you might wish to shut down so that you can transport it, or if you won't use it for a long while.
Also, shutting down completely wipes away the computer's RAM (random access memory), which might have some corrupt/junk data left in it from various things the computer does. It also lets the computer's components cool down. I definitely recommend shutting down at least once a month or so.
However, when you shut down and start up your computer, the hard drive and other components get more wear and tear, and use more energy.
I recommend all Mac users to sleep their computers unless they won't be used for more than a day or two, and all Mac users should shut down their computers at least once a month, just to give the computer a little time to clear its head.
<aside> PC laptops are a little different - because MS can't get its act together, PCs 'hibernate' - like a really deep sleep. The hard drive is accessed a lot during the 'hibernate' and 'unhibernate' times, causing a lot of wear and tear and much energy usage. This also takes a lot longer than the Mac way. Some PC laptops have different sleep modes that work more like a Mac's sleep... but it's usually best to shut down anyways, since Windows doesn't have the same ability to stay 'up' as long as a Mac. </aside>