Comparisons Betwen Deck-Building Games
Okay, I'll give it a shot.

First, what they have in common:


Static Cards: They are all static (meaning not CCG) card games.  Some (Dominion and Thunderstone) have expansions, but none of them require collecting or hunting down 'rare' cards to make the game better.
Victory Points: They all use a Victory Point system to determine the winner.  This is called "honor" in ascension (I think) but in essence, its still victory points.  The person with the most at the end wins.
Flexibility in numbers:They are all flexible on the number of players: Ascension is 2-4, Thunderstone is 2 - 5 (or 6) and I think Dominion is 2 - 6.  Not sure on the numbers exactly because Thunderstone has a nice single player variant, and both Thunderstone and Dominion can be expanded to include more players by playing with expansion cards.


Now the differences.


Deck Building:


Dominion is focused on building a deck to buy victory points.
While there are cards to interact and "attack" the other players, the game is centered around building a deck that will allow you to buy the most victory points.
Thunderstone focuses on building a deck from a general pool of cards, and then using your hand to "defeat monsters in the dungeon", thus gaining your Victory Points from the defeated monsters.  Some Victory Points can be purchased as well, but not usually enough to win the game, so defeating monsters is important to winning.
Ascension is much more like Thunderstone than Dominion.  In Ascension, you must also build a deck to defeat monsters to gain Honor (Victory Points) but unlike Thunderstone, the cards in the general pool are revealed one at a time.  So you have a line of 8 cards, and when one leaves play (your deck, or the discard pile) another fills that spot.



All are fairly easy to pick up, especially if you have played CCGs before or one of the above.

Dominion is probably the most well known and most played, as it's been around longer and there are several (many...) expansions.

I like them all and while I played Thunderstone first, then learned the others, I think that they all can be quite fun.  Most game shops will probably have Dominion on their "demo" shelf.  Several around here have Thunderstone and Ascension as well.