It all depends on your situation really. If you have the chance micing an amp can sound great. There are a lot of speaker sims out there that can do a pretty good job though.
I'd look at these factors:
1. What rig are you using? Tube amps almost always sound better when cranked up a bit. Solid state and modelers can sound pretty good at low volumes. And for quick and easy, it's hard to beat a modeler direct. It's not quite as sweet as a mic'd cab but it's also ten times easier.
2. Where are you living and can you crank it up a bit? If so, the moving air of a mic'd cab is really hard to beat. If you live in a small apartment (or with sensitive neighbors or record at night or with sleeping kids or whatever) you may very well be better off with a direct approach.
3. What recording gear are you using? If you have a decent mic preamp (either from a mixer, sound card, or standalone unit) then mic'ing an amp should sound pretty sweet. If you are just running into a cheap simple solution then the difference will probably be not as obvious.
4. Do you have the experience or patience to learn to mic cabs? Learning where to get the sweet spots and at what volume and with what combination of gear can take some serious time. You don't have to be a seasoned veteran but if you're new to it you'll need some trial and error time.
I have a little home studio that I just started using out for commercial stuff this year. I've recorded a ton of guitar parts both direct and mic'd. Right now, I use a Line 6 Vetta for a lot of it. Whenever possible I mic it up using a Mesa Recto 4x12 cab. For mics, the Shure SM57 is pretty much the industry standard for micing an amp. I use it quite a bit. My current favorite right now though, is the Sennheiser E609. They're both under a $100 and both are great at micing an amp.
The whole key to micing an amp is experimenting. You can read or hear about the ideal micing positions and distances but to really find the sweet spot you just have to experiment.
Here's a crazy example. I found (through trial and error) that each one of the four 12" speakers in my cab seems to sound better and certain applications. For tight chunky distorted rhythms the bottom right speaker just owns. Why? Who knows? Could be slight variances in the wood of the cab, in the speakers themselves (all are stock vintage 30's) or whatever. The top left handles cleans better. And I mic them at different positions and angles.
If you can email or pm me with the details of you rig I'd be happy to answer any questions I can that you may have.
Have fun man, a cool recording always makes my day.
D.
