I think the answer is A-Rod, hands down. Soriano put up great numbers the last two years, and 2B is a thin position, but remember, he's 28 now and not hitting in quite the potent lineup. He should jack a few more homeruns in that bandbox, but he'll also get pitched around.
Pujols is great too, but his position eligibility limits his value. A-Rod plays a thin position, and has proven that 50 HR, 10 SB, .310 AVG Seasons are the norm for him.