What is the difference between these two when selecting a video editing monitor? Which is better/worse and why? Thanks

the SMPTE is suppose to be "color accurate" to go along with your SMPTE color bars -- i haven't check awhile (2 years) but was approx 25-40% MORE (depending on manufacturer) for SMPTE monitor VS. P22 ... also i SMPTE usually has more resolution ( both are beyond SD tape formats resolutions) .. from what i remember SMPTE approx 800 lines rez ..P22 approx 600 lines rez ....
if you want to see the full resolution hook up a 2/3 CCd camera head that has 700-800 lines and you will be quite happy ...

DV25 looks excellent on either ...

I've written about this on several occasions, so you may want to do a search. If the client is going to see the monitor use the P22 phosphors. P22 has more the look of consumer monitors and will be more appealing to the client. If your work is for critical color correction or is for network broadcast get SMPTE-C phosphors. Client won't like the look, but they are more accurate.