blocks could be a bit restrictive unless you reliably download the same each month so you have predictable usage. if you ever deviate or find yourself having to hold back so you dont burn up your block, an unlimited account may be better and then you dont have to worry. unlimited is cheap enough now.
Usenet is considered decentralized, it is made up of many independent servers that talk to each other. take one down, and the rest continue on without problems.