I have a Boogie Formula Preamp, the one that was designed for recording direct and it does a fine job of it. This is how i usually record and it makes the wife very happy (that 4x12 cab I use can get pretty loud)but there are times when i mic it. What is the differance in the sound? That is going to depend on your set up. In general I get a little cleaner sound direct and a little warmer with mics. This is probably caused my a comination of the amp,speaker,mic,preamp comination I use. I can go in and add a little extra compression and eq it a bit to make the direct very close however.
Try it both way's and see what works best for you.