Apache Avro vs Messagepack performance 2013

Apache Avro vs Messagepack performance 2013

I have been reading a lot about Avro and Messagepack these days. Not sure
which one to use when?
I have also read post on the stackoverflow but nobody has explain when we
should use Apache Avro and when we should use Messagepack.
Currently, what we are doing is - we are serializing the JSON document
using Jackson and then writing that serialize JSON document into Cassandra
for each row key/user id. Then we have a REST service that reads the whole
JSON document using the row key and then deserialize it and use it
further.
Now we were thinking to have more compact representation of data as
compared to JSON and then write that serialize data into Cassandra. So not
sure what benefit, I will get, if I am going with Apache Avro or
Messagepack?
Our JSON document will look like this for a particular column family-
{"lv":[{"v":{"cateId":222625,"steqId":77111}},{"v":{"cateId":29322,"steqId":7711}},{"v":{"cateId":222619,"steqId":444477}}],"lmd":1379213614465}
But the properties and its value inside the above JSON document will get
changed depending on the column names as we have the metadata for that.
Can anyone provide some sort of suggestion which one I should go for? And
what benefit I will get if I choose Apache Avro or Messagepack? And which
one is better in terms of performance as well?
Thanks.