I went with the 2500# springs. Less than $100 (including shipping and tax) for all 4 from etrailer.com. I already have the Lippert shock type equalizer (stock on the trailer) and I have a heavy shakle kit and wet bolts that I'll install at the same time.
4-2500# springs gives me a total of 10,000# and the springs only have to carry 6800# of the trailer weight. That gives me a 3200# cushion. I think I'm comfortable with that. I figured any more than that and I would really start bouncing things around in the trailer.