I preffer buying 20's. deck packs have 1 guarantied 5 star card in it so that's more certainty.
and with buying 20's it just depends on what clans you are going for or what cards you are searching for. i personally preffered danger for a long time and ofcourse new blood when new cards come out again
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.