I work at an alternative energy research lab, so I've done a reasonable amount of experimentation and reading on the subject.
First, from the hard drive's perspective sleep is functionally identical to powered down--either way the platters aren't spinning and the heads are parked. Only difference is whether the drive's circuitry is partially powered or not, and I can't think of ANY way that would realistically affect lifespan of a drive.
Now, spinning a drive up and down may, in THEORY, reduce the lifespan of a drive... but at this point I don't think it's a realistic problem. Thing is, modern drives (particularly laptop ones, since they are expected to power down as often as possible) are built with the intention of being powered up and down repeatedly. In fact, for a while there at least, Hitachi/IBM was saying that their consumer drives were only rated to be on 8 hours a day.
If you were spinning the drive up and down once a minute, that would probably be bad for it. But in practical use I don't think you'll likely notice it--heat, more than mechanical wear, is what generally kills drives these days. If you want anecdotal evidence, my first-gen G5 tower has been woken and slept at last 5-10 times a day since I bought it (over 4 years), and neither of its two hard drives have failed. In fact, the stock drive is my internal backup and is set to sleep on inactivity, so it probably goes through a couple dozen power cycles a day.
In the lab I manage, the computers are mostly set to sleep the drives when inactive, which amounts to a dozen or more sleep-wake cycles, and out of around 20 machines I've seen two drive failures in the past 7 years, both OLD Quantum 10GB units that had been in service for around 5 years.
So the bottom line is, don't worry about the effect of "normal" sleeping of your drives--it may affect life, but not enough to worry about.
Second question: Depends a lot on the computer.
On a laptop, sleep is VERY efficient, since they're designed to last quite a while in a sleep state on the battery. It uses slightly more power than shutting down and unplugging the power brick, but not really worth bothering with in my opinion, and I'm extremely environmentally conscious and energy-aware.
Desktops are a different matter; older desktops generally draw a fairly large amount of electricity even when off, and actually pretty much the same amount when asleep. My old G4 tower, for example, used about 8W when "off" but plugged in, and maybe 10W when sleeping. So there was almost no point in shutting down unless you were going to unplug it or turn off the power strip (which isn't a bad idea as a means of avoiding parasitic loads, actually).
The G5s, however, are built differently--there's some sort of contactor in the power supply that reduces the "off" power draw to very nearly zero, so there IS an advantage to shutting those down even if you don't turn off the power strip.
I haven't gotten my hands on a Mac Pro to measure yet, but I'm guessing they're similar.
Bottom line: Don't worry too much about sleeping your laptop whenever you're not using it, not really worth powering down a laptop versus sleep, but if your desktop is going to be off for more than, say, an hour, it may be worth shutting down to save some energy (particularly at night), and you can save a little extra by turning off the power strip.
