As a player Sampletank is dated and very hard to read if you do not have 20-20 vision. The content, while not being the cutting edge is good and useful. Sampletank 3 is long overdue.
I am not familiar with the EastWest player, but its limited to their propriatory content.
While there is a vast about of third party content for Kontakt much of it will not work with the player. The will only work in player if the developer has payed a fee to NI, inevitably this excludes the free and low cost content. Requirements need to be read carefully before purchasing.
That said the Kontakt player is very versatile with vast array of built in effects and editing options. Features wise it wins hand down.
Once you have the full version of Kontakt the third party content available is huge. There is some fantastic stuff for free or just a few bucks as well as sophisticated high end libraries.