Have you considered a 3rd option of doing a Microsoft AD Import? I implemented Domain Browse List for my Altiris 6 server which works fairly well to find "online" systems and pulls in OS info which I needed to build some of my collections. Only thing is that I had to schedule resource discovery to occur more often since I couldn't expect all PCs to be on at any given time on my network.
As andykn101 mentioned, Domain Membership will pull in all machine accounts whether active or not (i.e. you'll end up with a lot of junk in your DB if you're like us where we have tons of stale machine accounts in AD) and, strangely, doesn't gather OS info so this wasn't an option for me. Altiris docs indicate Domain Membership is for larger environments and Domain Browse List for smaller networks but because domain membership doesn't pull in OS info, I had to go with browse list discovery.
For my SMP 7.1 SP2 server, I decided to go with an AD import since you can specify criteria to include only enabled workstations which I wanted and it also pulls in OS info. This was the best option for me and I can do an off-hours discovery/import from AD instead of periodically hitting the network during normal work hours. Of course the resource discovery method you choose depends upon your needs and environment.