Content Filtering vs. Layer7 Filtering
Which one is better method to use?
It depends on what you are trying to do really.
http://wiki.mikrotik.com/wiki/Manual:IP/Firewall/L7
http://wiki.mikrotik.com/wiki/Manual:IP/Firewall/Filter#Properties
Content matches on a per packet level if I remember correctly. So if content is split between 2 packets it will not match.
Layer 7 matches the first 10 packets or 2 KB of a connection. So if the content you are looking for exists out of that initial data, it will not match.
You need to be very careful when using these features as they are very CPU intensive, and will greatly affect network performance if used improperly. Neither are designed or intended to look at every packet in a connection, so don’t expect to use it as such. Also do not expect to use these features to block certain websites, as they are once again not intended to be used like that.