These sites should explain things well:

http://www.rc-submarines.com

http://pyerok.club.fr/Ballast_EV.htm

While I can probably say that I prefer static diving the most, I can't exactly say that dynamic diving is worse. Each method has its own advantages:

Static: Scale factor, your sub dives and surfaces a lot like a real sub. Can be a little expensive to build but the results are worthwhile in the long run.

Dynamic: Positively buoyaant, therefore, you have an automatic failsafe intergrated right into the design. Certainly cheaper to build, a good choice for most beginners.