I am a former JSTARS ABM, leaving the wing just before it became a Guard/AD unit. From the aircrew perspective, the JSTARS NAV position is something of a hybrid. First, the NAV position itself is actually back in the mission crew area (first console). The console is the same as other mission crew consoles with the addition of some flight instruments on the side. This means the JSTARS NAV has access to the same data, imagery and information as the rest of the crew. The NAV mission also involves more than getting the aircraft to tanker, then to the orbit, then back to base. On JSTARS the NAV works very closely with the onboard Intel Officer/Tech on defense of the aircraft. In this area, the NAV works directly for the Aircraft Commander and the Mission Crew Commander. So in some ways, the NAV is both Flight Crew and Mission Crew. One note of warning: The mission crew is primarily made up of ABMs. They start as controllers, moving to sensor management and battle management before becoming mission crew commanders. NAVs have not been selected as MCCs so if you are a NAV, you'll always be a NAV unless you go to ABM training like several of the 116th's WSOs did when they tuned in their B-1s for the E-8 JSTARS. NAVs have been commanders however (including squadron, ops group and wing commander) so they are recognized favorably in the Wing and in the USAF.
Bottom line, If you are going to be an ISR NAV, I think the JSTARS mission is more robust than say the E-3 or RC-135 because you are also part of the mission crew. Those who have flown ISR aicraft will testify to the distinct differences between the flight and mission crew aboard their systems. I have over 3000 hours in the USAF and NATO E-3 as well.
OK, having said that, you should know the downside to flying with JSTARS or any other ISR platform. Basically, you fly loads of hours but don't actually go anywhere. You take off, hit the tanker, then head to the orbit. You then fly a standard pattern for several hours and go home, or back to tanker if your mission is long enough. Missions routinely last from eight to fourteen hours but could go past twenty hours if required. My personal record was 18.5. When I left JSTARS the record was 21 though I think it may have been broken. You deploy to all the same places, all the time... There is very little variation in your schedule. Finally, all ISR systems including JSTARS are known as High Demand/Low Density (HD/LD) systems. As a result, you are gone all the time.
If you want to travel and do something more diverse, then C-130s are probably the best option. You should also note that while I was at JSTARS we got NAVs from all over the USAF (B-52, E-3, RC-135, EC-135, OC-135, KC-135, etc). It will probably be easier for you to go from the C-130 to the E-8C when they switch to the J model than vice versa.
Just my thoughts...