My experience, and more so, my research, has indicated that they both have good effects on raising the 'boiling point'.
Each pound of cap pressure raises the boiling point 3.25-degrees X cap pressure. So, if you're running a 15-lb. cap, then you've effectively raised the boiling point of your system 48.75-degrees or up to 260.75-degrees.
http://www.innerauto.com/Automotive_..._Pressure_Cap/
http://www.ferret.com.au/articles/66/0c044c66.asp
http://autorepair.about.com/cs/cooli.../aa081101a.htm
And all you never really wanted to know:
http://www.jcna.com/library/tech/tech0011.html