my two cents
Tucci,
This is pure opinion, but I base it on my experience.
First off, I think the answer depends on where you are in the process and the facts of your case. In my opinion, the earlier you get assistance in the process, the more impact your representative/attorney can have on the outcome. A great attorney will review your case file, medical history, and discuss your case with you and then analyze the likely and possible outcomes. They will then discuss any additional evidence that could justify a higher rating than the evidence currently warrants. They will assist you in gathering the evidence, and insure that all the procedural requirements have been followed. If done early enough, this can result in you getting a better result at the Informal PEB than you would have otherwise. If you end up disagreeing with the IPEB, they will then present your case, and argue for the result you want (as long as it is realistic). All the while they will be watching for legal error and preparing mentally for appeal should the PEB get it wrong (either factually or legally). If the Formal PEB does mess it up, they will draft a strong appeal stating the factual and/or legal errors in the process.
Contrast this with getting an attorney a few days before the Formal and basically being stuck with the evidence as it exists. The attorney who starts your case a short time out for the Formal PEB is greatly hamstrung in shaping the outcome. They can still present a good argument, but it is much harder to get a good result. They are basically stuck with what is in the file at that point, plus any relevant testimony you can give.
My opinion of the DAV depends on which PEB you are going to. In San Antonio, there is a great DAV rep who is extremely knowledgeable in the VASRD and does a very good job arguing cases. I would say that most DAV reps should be more familiar with the VASRD than many JAGs because they tend to have more experience with it. The downside is that they don't have legal training, so may not be able to identify all of the legal errors that the PEB may commit in your case (if any). You can use their services at the PEB, however, depending on where you move when/if you are separated, you may end up with a different VSO for your VA case.
So, if you ask me, good civilian representation is best (mostly because of their ability to start early on your case and to spend more time in developing the evidence...but also because they can help you in subsequent appeals). Downside is the fee and its sometimes hard to gauge their level of competence (true of all attorneys, though).
Next, I would give a slight edge to the JAG, because of legal training. However, depending on several factors, DAV may still be a good choice. If the JAG is inexperienced, or if you have an exotic condition that could arguably be rated differently under the VASRD, I might recommend the DAV, especially if in San Antonio.
Final caveat. If the facts of your case do not justify a higher rating than 30% under any circumstance, I would probably skip the civilian attorney and go with whoever you feel more comfortable with, JAG or DAV. All in all, for anyone, I always advise talking to a few different representatives (attorney and/or DAV). Go with whoever you think is best.