$20000, or even $5000 will go a VERY long way in buying fuel...
While both the $20K for a Hemi conversion, AND the $5K for a supercharger would provide more power levels, neither are IMHO going to provide a very cost-effective alternative to what you have.
While on the surface, the $5K for the supercharger seems nice, that V6 engine was not designed to handle the increases in compression the supercharger creates. The entire engine structure, (pistons, connecting rods, bearings, block, cylinder heads, and cyl head bolts, crankshaft, etc. etc. etc.) are simply not designed for the increased loads that a supercharged induction system creates. May work fine for 5-10 thousand miles, but if you are planning on keeping this vehicle long-term, it is my humble opinion that you are better off leaving the base powerplant alone.
BUT... If you are absolutely convinced that you do not have sufficient power, AND are willing to spend $$'s to get more, the better of the two options is a Hemi, including the trans that will be needed to handle the increased torque.