1U vs. 2U

We are about to move forward on purchasing a few poweredge servers to add to our existing rack of servers.  I am basically looking at the 2950 III (2U), 2970 (2U), and the 1950 III (1U).

It would be ideal, if we could add 1U servers, which increases our rack capacity, so, I have some general questions.  In comparing the 1U and 2U servers, what are the PRO's and CON's of going with a 1U over a 2U? Examples: Max Hard Drive Space (RAID 5)?, Heat?, Noise?, other issues?

thanks...