A 3D powerplant with G2 Low Emissions, no special effect, will have an efficiency of 0.49 while putting out 8.5 MW, weighing in at 2.16 tons.Thing about exploration ships is that they usually don't need a lot of juice so a PP that provides enough will probably produce a bit extra too.
So, G1/G2 Low-Emissions mod' will get you a seriously cool-running ship and then you can apply the Stripped-Down XFX to reduce the weight to less than the standard PP.
Don't go higher than G2 with the LE, though, or the extra weight won't be negated by the XFX.
A 2A powerplant, stock, will have an efficiency of 0.4 while putting out 9.6 MW, weighing in at 2.5 tons.
I'd go for the 2A powerplant for the headaches it'll save me despite weighing slightly more than the 3D powerplant.