here is my ranger towing my boat:
http://www.explorerforum.com/forums/showthread.php?t=213737&page=7
how big is yours?
how bad is your sag? do you have much rake when the truck is unloaded?
if it sits flat unloaded then the helper springs set to activate with any sag, then that would be the best bet.
if you have rake, then i would either set up helpers, or timbrens to activate when it sits about flat.
my truck sits nicely with nothing helping it out with the boat, but when i have a load of metal or something for work it sags, and im considering setting up something to assist right about when it hits zero rake.
anyway, ive never driven a vehicle with helper springs, but ive heard good things about them.
at work we generally recommend timbrens, just because they last forever, zero maintenance, and they can be set up so that it rides nicely unloaded, and then with a heavy load they help alot. my boss has them for his camper, and they are pretty great.
good luck!