The differences between the two programs - Time.
With Herolab, you are paying someone to make the Books in the digital format in a timely manner. Hence, their sets are made faster since they can pay someone to focus on that - 8, 10 or 12 hours a day.
With PCGen, it's the volunteer who is donating their time to make the Books in the digital format. Since volunteers normally have day jobs, the sets are a little longer to get.
See, TIME. One you pay for the time to get it faster, the other the cost is waiting for someone to donate the time.
As far as useability, I think both do a decent job. I'm biased though as I work hard hours to make PCGen better in little ways on a daily basis.
PCGen is a volunteer community who give back to the roleplaying community. The more people who join and contribute the better it becomes. I also like the fact that I can alter any aspect of the data to fully customize my experience - which supports my homebrew games much easier. I'm not sure if you can do the same with Herolabs stuff. Support - both have decent support. If you come on our Yahoo Groups, you'll either be talking to me or one of the volunteers. Our Mailing Lists & Forums are very active.
Okay, back into lurker mode so I can get work done.
Cheers,
Andrew Maitland aka Legacyking
PCGen Admin Silverback