whats the difference in os 91 engines?

[nathan211]

im looking at getting a trex 700 this winter and am not sure which engine to get. I understand that some are pumped and others arent. Is a pump really necessary? So not really sure which is better the one with the black head or blue? Whats everyone running?

[John Cook]

OS HZ non pumped
runs great, and one less thing ie pump to worry about, plus it's a bit cheaper.

YS 91SR rox too