Hello and welcome
Personally i would go for the 2.8Ghz northwood over a 3.0Ghz Prescott.
For example over the last few months i have compared a 3.0GHz Prescott revision "C" (not very good overclockers compared to D and E revisions) which i could only hit 3.4Ghz stable and that was with the load temps in excess of 60*c.
As a comparison i bought a cheep Northwood off ebay for Â£50, a 2.4C revision M0 which performs much better than the old Prescott.
I am currently running it at 3.6Ghz and its still 10*c cooler than the prescott at stock speed/voltage.
Yeah you are right in saying that the northwoods have a 20 stage pipeline compared to the prescotts 31 stages.
This means clock for clock a northwood is faster than a prescott, But with the additional steps in the cpu the Prescott can scale higher (hence prescotts at 3.8GHz stock where as the northwoods topped out at around 3.2GHz stock i believe)