IMHO, you can't find a better made case than Vaja. That being said, they are expensive and do add bulk to the Treo. It all depends on how you use your Treo 650 - and your needs in terms of size. I'm in the process of doing a review of the T66 by Vaja -- I dropped it my Treo on concrete and was VERY scared.
The Vaja absorbed the entire hit and my Treo is fine. With a skin case, you are getting protection against scratches more than against drops. I haven't used either of the skin cases you referenced. With any open case, I'd recommend investing in
Screen Protectors. I never worry when using an open faced case, because I'm using a screen protector. This allows me to throw my Treo in my pocket and not worry about scratching the screen.
I've owned both Skin Cases and a Vaja case for my iPod. After a few months, this particular skin case looked kind of ratty. I probably could have washed with warm water/soap, but I decided to go with a Vaja from the Apple Store. After almost two years, a little leather cleaner and the Vaja looks great. Same thing -- I've dropped my iPod on several occasions and it's still humming.
So, I guess it would depend on preference and everyday use. If you're using a belt clip, I'd say you wouldn't notice the added bulk of the Vaja. If you're not clumsy (like me), then a skin case will offer you protection against scratches, minimize the size added to your Treo and allow people to actually see your phone! :D
-Chris