First, I think that all of the materials are a good investment - both instructors clearly put a lot of work into the products, and there is a lot of good information in each. That said, Joe's are certainly more up-to-date (being for messiah:animate rather than the plugin version), and probably represent a better starting point for new users. For details, read on!
Joe Cosman's "Character Rigging" CD and John Riggs' "Beginning Character Skeletons" DVD/VHS are pretty similar products, but there are a few differences. First, John Riggs does spend a lot of time and effort on organization - he takes a lot of care to group all of his animation controls, effects, meshes, etc. under their own Organizational null, so that each can be completely closed when not needed. However, due to the tutorials being done for the plugin version of messiah, he also has to spend a lot of time going into Setup Mode and linking nulls into the skeleton, which is no longer necessary in messiah:animate or studio.
The rig you end up with at the end of "Basic Character Skeletons" is just that - pretty basic. It does give a good grounding in the process of rigging in messiah, but it uses only the most basic IK setup on the legs (no reverse foot), and no IK on the arms. It does cover straightforward ways to attach accessories to characters with the MoveTo expression, however, which can be handy.
Joe Cosman's "Character Rigging" CD is, as I said, the better starting point for new users these days. While he doesn't put as much time into the organization of the rig as John Riggs does, the rig you end up with at the end of the CD is a more functional, sophisticated rig. Topics like arm IK, proper deformation of the forearm and upper arm, and the reverse foot setup are covered in detail, while you'll have to buy Riggs' "Advanced Character Techniques" DVD to get the same information.
I like the fact that Joe's tutorials are WMV files - you can copy them to your hard drive to view them whenever you want without carrying the CD around with you (very handy on a laptop), and because you get them on a CD, you also get objects and scenes that you can use to follow along with the tutorials. Finally, Joe has encoded his tutorials at 800x600, which means they are fairly clear and easy to follow. Because Riggs' tutorials are on DVD/VHS, they were recorded at a fairly low resolution (I think the actual size of the Project Messiah interface in the videos is about 600x400), and he has to "zoom in" to specific areas of the interface (like the expressions list) to make things clear.
The only problem I've had with the rig that Joe guides you in making is the hands. Specifically, Joe has all the bones in the hand parented to the IK Goal for the arm, and while it is an elegant setup, I've had problems with it while animating. The biggest issue is that, because the hand orients to the rotation of the goal null, it ends up effectively rotating when the null is translated to move the arm. For example, if you move the goal null in the Y axis to lower the arm from a straight-out position, the hand stays oriented to the null - which hasn't rotated - and by the time the arm is at a 45 degree angle to the body, the hand is at a 45 degree angle to the arm. So, whenever I animate the position of the arm, I have to re-orient the hand's relationship to the wrist. It's possible that there's something I've missed in the setup somewhere (perhaps in the expressions order), but unfortunately, when Joe goes over all the freedom the rig gives you at the end of the tutorial, he doesn't move the hand, so I can't be sure.
For face setup, there really is only one product available, and that's Joe Cosman's just-released "Rigging the Face" CD. While some have questioned its price in relation to the body rigging CD, since the included videos are only about half as long as those on the full body CD for an only "somewhat" lower price, I think the quality of the information easily justifies the price. You get pretty decent coverage of a full face setup using a combination of bones for tweaking expressions and morphs for phonemes and such. Also, Joe goes into some detail on how to generate morph targets - he uses Lightwave, but the information should be useable by anybody, regardless of software.
The only thing I was hoping to see and didn't was coverage of methods of handling full-body characters with morphing for the face - there's something basically unsatisfying about loading multiple copies of a full character just for the facial morphs, and since the CD only covers the face separate from the body, it really doesn't cover this. I know this is something that Puppetmaster can handle, and was hoping to see it in action.
Finally, there's John Riggs' "Advanced Character Techniques" DVD. As mentioned above, this goes over such things as the reverse foot setup and using expressions to get a nice deformation in the forearm, both of which Joe Cosman covers quite well in his "Character Rigging" CD. However, there are a few more techniques covered that make the DVD worthwhile - things like slider and expressions setups for hands and some really cool uses for the InheritRoot function for making automated but controllable tails, wings, and spines. This is the kind of stuff that will make even the worst math-phobe want to learn some expressions, and Riggs does a good job of explaining how the functions work and why.
Overall, as I said, I can whole-heartedly recommend all of the products mentioned. If you already have the Riggs DVDs, I still think that Joe Cosman's "Body Rigging" CD is worth a purchase, and you really can't go wrong with his "Rigging the Face" CD. If you already have Joe's CDs, you still might want to take a look at Riggs' "Advanced" DVD for some cool ideas not covered by Joe.