That was an easy question. You can have different compression levels for different partitions of an index and thats why you need to look at sys.partitions. You can also find the number of rows for each partition in sys.partittions. If you want to quickly find out how many records you have in a table, sys.partitions is the fastest source.
MCITP:Database Developer 2008
MCTS: SQL Server 2008, Implementation and Maintenance
MCSE: Data Platform