Speaking generically, I will note that all pitchers (whether RP or SP) use the same formula to determine OVR. And that fomula incorporates DUR/STA (i.e. how many innings he can pitch).
As such, if an RP and an SP have identical ratings but for the DUR/STA, the SP has the higher OVR and is worth "more" than the RP, simply becacuse of the additional innings at itentical effectiveness
So, going the other way, if an SP and an RP have identical OVRs, the RP by definition has to be more "effective" than the SP, with the SP making up the gap by being able to pitch more innings.
In terms of trade value, assuming that OVR was an effective calculation of total player skill (most would mostly yes, but with some glaring exceptions), then an SP and an RP with the same OVR should have the "same" value, and in trading, you are deciding between more effective for less innings (RP) or less effective for more innings (SP)
and note I use the OVR as a way to ensure the players being compared are somewhat similar because oboviously, even if a SP could throw every inning all season (100/100 DUR/STA) he has no worth if he can't be effective (bad splits and pitches). And the same goes for an RP who never gives up a hit, but can only pitch 1 IP a season). So the trade-off between effectiveness and innings has a limit