none of you have actually addressed my 2 explicit concerns:
1) power usage? (is one much higher than the other?)
2) longevity of device? (is one more likely to outlast the other if they are on 24/7?)
the portability and built in screen on a laptop is a plus, but not a showstopper.
since the device does not have to be placed in a really tiny space, i dont need a really tiny beagleboard type thing; in terms of size, a mini-itx would be fine, as would a laptop.
Well, it's a tough thing, hehe. Here's my best answers:
1: Generally speaking an equivalent laptop hardware-wise is going to use less power per component because they're all optimized for super low power, *including the PSU*. So an Atom-based laptop or netbook will generally use less power than a desktop of equivalent specs, *especially* one with a separate LCD monitor. That being said, if you go for all low power desktop components, you can get close or perhaps even beat a laptop, with a carefully configured build.
The average laptop uses between 10 and 20 watts of power at idle, some less. That goes up to 30-60-ish when under full load, depending on the laptop and CPU. When charging and running at the same time, it's more, but that's not likely to be a common thing in your usage scenario.
Atom-based netbooks/laptops generally use even less, of course. Idle is 5-10 and load is up around 20-25. e.g. http://reviews.cnet....-2.html?tag=txt;page
Asus Eee is the stand-out here at least than 20w at load:http://reviews.cnet....-2.html?tag=txt;page
Remember also that these numbers include the display! A comparable desktop, even super low power, will use similar levels of power *without the display*. Cnet actually has some good power usage info in its reviews, as you can see above, and they do desktops too. Here's an Atom-based desktop review and you can see the power usage is actually fairly comparable, but again remember this is without the monitor:http://reviews.cnet....-2.html?tag=txt;page
Here you can see an all-in-one Atom-based system with very low power use, and it includes the monitor of course. Still more than a netbook, but the screen is probably larger, so that may account for some: http://reviews.cnet....-2.html?tag=txt;page
2: Unfortunately point #2 contradicts #1 which is that laptops tend to be less reliable over the long term *and* are more expensive and more difficult to fix if they do break (generally speaking). Still, most of the wear of a laptop is in moving it around and having exposure to lots of dust on a regular basis. If you keep the air paths clean, keep the bottom and sides well ventilated, and don't move the laptop around much, it has a good chance of surviving for an acceptable amount of time.
As for size, it's not the fact that the small boards are small that makes them appealing, it's just that a helpful byproduct of size is generally lower power use as well. Hence the mini-ITX approach vs. full-size ATX. The tiny size is not a requirement, but smaller size does enable as well as generally go along with lower power use in the market.
If it were me I'd get the laptop just for versatility, unless you already have one, or you really want to tinker with a hardware project. The laptop option will generally be faster and easier and get you closer to the results you want in short order, but the hardware build would be more challenging and potentially enjoyable. If it's the software side of things you really want to tweak, the laptop will give you the quickest, easiest platform to do that.