I would prefer to never have an IV started en route, but I would trust any medic I have worked with so far to do it. I would prefer a medic, given my limited experience but I know RN's are quite capable, and I have seen some good ones.....I've also seen a lot more that werent as good.
But in defense of RN's, as I stated previously, if it is an ALS run for the service I have been at, the pt gets IV access, so they dont get as much experience and practice IMO.
ETA: Again my post sounds weird so I'll try to clarify. Rn's here work 8 hour shifts, whereas medics work 12's. In that time a nurse may start 1 or 2 IV's, or if at one particular hospital, they will have a staff medic do it so they may start none. A medic in the field will start ~8 on a normal day