Depends on your collection, your play style, and how much real money you have.
If you are after the entire collection, and you have like 25% of it so far, 50 cred pack
is good choice for you.
If you are a competitive player and just want certain cards AND you have reall money to waste, 20 cred clintz pack
(although in general this pack
is useless -- Clintz have to be upped in this pack) is for you -- since you really need to buy your cards on the market.
If you have a somewhat complete collection or you are looking for specific duplicate cards then market AND specific 20 cred Packs
are for you.
New Blood is for everyone really, whenever a new card comes out.
There are more best practices to buying packs
and any AOD members is more then free to ask me to go into detail -- in our forums.