B&M Transpac (not the just the 'shift improver kit'. This kit gives more options like full manual control at all times (if you want it), full manual control in 1st or 2nd, but automatic in Drive, or just shortened shift times. You make the call.
Go with the TransGo.....the B&M kit gets it firmer shifts by just increasing trans pressure, while the Transgo Shift kits actually address the reasons why the shifts are not firm in the first place.
You get what you pay for and that is why the cost is vastly different. The longevity of the transmission being worked on is the key to what you should do with it.