What would I get for 1.3 more ounces?

I haven't a clue!
I dunno that stuff.
But, I can make a recommendation to help you decide. Put on the jacket ya got and have someone spray ya down good with a garden hose. If it seems to work OK, roll with it and save the coin!

1layer fabric, 1layer membrane, .5 layer over coating to protect the membrane = 2.5 layers

The fabric used in the Plasmic is eVent DVL. It is simply rebranded under the DryQ umbrella by MHW.

It will breathe noticeably better than the Helium's Pertex Shield Plus which is a monolithic polyurethane membrane, which "breathes" by absorbing and releasing condensation. eVent is an ePTFE membrane, which is porous and air permeable, so humidity is pushed out without state change (vapor to liquid to vapor).

It gets more complicated than that, though. Both membranes are quite fragile due to their thin coatings and so with even casual use you may find the need or desire to test the warranty policy of your jacket's manufacturer. By that alone, most people would agree that Outdoor Research has a far more generous warranty coverage.

So the MH breathes better. Is it also more waterproof?

I own the Helium II here in the PNW. It will wet out after 1 hour of constant rain. I bought it for when I travel to a place like SW Asia where I will get 15 minute downpours.
I know "Wired" recently used the same jacket on her CDT thru-hike.

> So the MH breathes better. Is it also more waterproof?

A loaded question. Water should not pass through any jacket like these. However, when a fabric is saturated with water on the outside, regardless of whether or not it is actually leaking you will get moist because wet fabric doesn't pass internal humidity well.