What will be the best data model for membership based websites

If we're talking about something where you may want to query on one or more of the attributes my suggestion would be multiple columns, indexed appropriately (for a small, infrequently-changing data set, index all the columns).
This works best for the case where you may want to run queries like
SELECT * FROM table WHERE attribute1=1 AND attribute3=2
since you will have several indexed columns and searches on one or more of them will be relatively efficient.

If we're talking about something where you only ever want to query on the aggregate (all attributes match) a single column will be more efficient in terms of storage, and likely in terms of indexing as well.
This works best for cases where the previous example would always devolve into
SELECT * FROM table WHERE attribute1=1 AND … AND attribute17=17
as rather than having to cross-check across multiple columns it will simply be
SELECT * FROM table WHERE attributeColumn=1234567 -- Rather than having to scan 17 indexes (or examine the contents of 17 columns in every row) you will just have the single index scan.

Cassandra is a Columnar Store NoSQL Database, Which goes beyond the Key Value pairs.

A Column family in cassandra is a container for rows, like table in any RDBMS.

And These are Stored and represented in Json Formats.

Below are some links where you can get more info:

http://wiki.apache.org/cassandra/DataModel

http://www.datastax.com/docs/0.8/ddl/index

http://www.javageneration.com/?p=70