Posted By:
Jeff OInteresting questions.... here are my opinions on some of the issues raised.
I think that 150 series cards are slightly easier to find than 1909 cards... but not a whole lot. The nice thing about 150s is that you can sometimes get them for a bargain, as the seller isn't always aware of the variation. Also, some 150s have slightly different cropping than their 175 series counterparts, so they're a little different.
With regards to scarce cards... it's difficult to say. I believe that all 1909s are equally tough. This is based on the fact that all three of the uncut sheets that I know of consist of exactly 76 cards - one of each player. This seems to indicate that there are no double or single prints, and that each is printed in equal quantities. As for the 1910s, two partial sheets appeared in Mastro auctions (6/24/99 and 11/16/00) and appear to indicate that there may be some double prints... but it's hard to tell. I personally have always found the Portland players particularly tough, but it could just be me. As for the 1911s, I am unaware of any sheets or scarcitites.
As for scarcity, I refer back to the American Card Catalog of the 1950s-1960s. During that era, value was generally based on scarcity of the issue. In the 1960 guide Obaks were priced at $0.30 each... while T206s were $0.10 and most other tobacco issues were $0.20 to $0.30. I feel these numbers have some weight, with Obaks being considerably tougher than the more common issues like T205 and T206. Mind you, the 1960 guide also prices T217 Monos at $0.35... and I will gladly pay a slight premium over the price of an Obak for one of these! 
Hope this helps...
Jeff