I go with switching my lights out yearly, and only half at a time . So, the first set got switched out at about 8 months. I've never seen a significant change in brightness over a year, but I don't have any equipment (yet) to check spectrum shift or exact PAR.
420nm bulbs are more purple than the 460nm. What that really means is you'll get more of a florescent effect out of the 420nm. It looks more purple than VHO actinics to me, though. I've found an equal mix of 420 and 460nm bulbs gives nice results.
What Vinnie said about kelvin. Track down GrimReefer in the equipment forum and talk to him about bulb selection. The guy's a lighting savant. Tell him what kind of look you want (bluish, white and crisp, good florescence, etc.), and he'll be able to suggest a bulb combination that'll give you what you want. I really don't know how he does it.
For what it's worth, though, for my 5 bulb setup, I went with 1x460nm actinic, 1x420nm actinic, 1x6500K and 2x12,000K.