The rye works well for me and I've never had a problem with killing it in the spring like some have proclaimed.Rye sends down pretty deep roots and produces a lot of biomass.
As for the eel grass, 4 pounds per square foot sounds pretty impressive. When I've dealt with it it dried out weighing next to nothing and blew around in the wind. Also it took a long time to become part of the soil food web. That was my experience with the stuff anyway..
Try the rye this year especially since you're making compost out of the eel grass.
Sort of the best of both worlds.
Since both are relatively inexpensive I don't think you can go wrong with either.
Good luck with your decision.

Darth weeder -- have you been able to kill the winter rye by just spading, or are you using machinery? I planted winter rye one year and it seemed difficult to get rid of, but I may have had it mixed up with the quack grass, which I didn't know yet that I had, so it might not have been the rye that was the problem. I have heard you need to till it and disk it both.

I have used Winter, Field, Cereal Rye as a winter cover crop for somewhere around 45 years and have never had a problem with it regrowing when cut, or knocked down, before seeds formed. The Rodale Institute has done extensive work on crimping Winter Rye to be used as a mulch and the USDA has now found this process to be something worthwhile, I would suspect this would not be something they would promote if regrowth were a problem.

Which to use will depend on how available one or the other is. If Eel grass is more readily available, and free, that is what you should be using. However, for others of us the Rye seed is more readily available even though we need to pay for it.

Here is a link that might be useful: Crimping Winter Rye

elisa,
I'm not sure about the confusion between quackgrass and winter rye. Maybe I don't know what quack grass is but the rye grows 5-6 feet tall. I've weed wacked it down, used a grass whip and a lawn mower to kill it.
All worked well.
Wishing nothing but success.

Quack Grass is an invasive, noxious, perennial weedy grass that spreads by rhizomes as well as seeds. For some of us it is the only grass we can grow, but is a pain in the planting beds.

Here is a link that might be useful: About Quack Grass