i personally prefer ngk and denso. ngk are just a good solid product, and i have seen densos last well beyond triple their service life, and they're what toyota use stock so they must be good :)
i don't think a $25 (EACH!!!) spark plug is going to make a lick of difference in any k motor, with its imprecise and inconsistent air/fuel metering and crappy combustion chamber shape
maybe if you were running an efi high compression twinky crossflow thing (100kw 4age for instance) you would have reason to use a decent plug
for a K motor, about the most r&d you'd bother doing would be to index the plug and do a few dyno runs with the electrode facing different ways, probably only good for 1 or 2 rwhp anyway
that being said, i'd never tried fancy plugs, so if anyone does and likes them, i'd be interested to see some definitive test results (ie: not the "butt dyno")