Whic mail server?

[Aflatoon Aflatooni]

Hi,
I am running a server that is acting as the mail server for only internal users (about 50 users). Currently we are running Sendmail, but reading on other discussions I noticed that qmail and other programs are suggested.
I am wondering if qmail is thought to be better than sendmail. Is there a matrix of features and functionalities that would compare the different mail servers? 
Any suggestions on spam filters like spam-assassin?


Thank you