Which foam for stringers

[JohnC]

I am rebuilding a 77MA 20 and I want to re-foam the stringers. I assume 4# foam would be the best bet to fill the box stringer but I'm not sure. US Composites has 8# available but I believe that's a but overkill. I believe the stringers will be around 12 cubic feet or less so weight of foam is no big deal; the cost of the foam goes up though.
The rest of the space under the deck will be 2#. I will be cutting as much of the top of the stringers out as is needed to remove the old stuff and then re-glassing.

I would rather ask then assume! Is there a reason to use one foam over the other? Thanks.