P5. I run a V-Spec, but if I had another choice, it'd be another 421B or a P5. The V-Spec makes crazy power out of the box but this kind of power doesn't last long- not like the Novarossi which takes some finesse to get broken in and tuned, but once broken in fully, makes awesome power and lasts multiple gallons. To get the V-Spec running good right out of the box, you're looking at:
$250.00 - VSpec
$75.00 - ConRod to replace after break in (Break-in is hard on OS conrods)
$55.00 - Ceramic Bearings; The stock bearing will give up after about .5-1 gallon. I've had 4 V-Specs and they've all given up at that mark.
So, that's right around $380.00. Decent P5 is $275.00, no fussing with anything after that.