The thing with Tomlinson is that he does more of the all-purpose yardage stuff...Alexander rarely makes any catches out of the backfield. Tomlinson had like 100 receptions two years ago!
Remember that Tomlinson has had a rib problem ever since the Oakland game, and I think that it affected him for this year.
Anyway, if you are in a PPR league, I wouldn't fault you at all for taking Tomlinson. And remember...without the huge TD numbers that he accumulates, Alexander would be Deuce McAllister. Of course, there's really nothing you can do about that except stop Alexander from getting those easy TD's.
I would also like to note that the top 5 RB TD performances have all come in the last 10 years (and the first four in the last five). Why is that when the NFL is supposedly a passing league?
(Sorry for the hijack...interesting question though)