Aaron,
I have a Cattarp2 that I got from a BPL member. I just used in last weekend and I like it quite a bit. The catenary cut makes it super easy to pitch. Here's a picture with my ptarmigan bivy:

My Cattarp2 weighs about 9oz minus the guyline and stuff sack. (This is using a non-digital scale with .25oz increments.) I use the BPL Aircore Nano line kit (.75oz in my config.) with 8 GG titanium stakes at 1.75oz for 11.75oz for the setup + stuff sack. Add a ground sheet at 1.5 oz, and the bivy at 7.5 and my shelter system is 20.75 oz, and totally flexible.
It's listed at 11.5 oz including guyline and stuff sack, so you can cut weight using lighter guyline. If Oware uses the same guyline for all it's tarps, then you could probably subtract ~2-2.5oz from the listed weight to get the weight of the tarp only.
I don't think the Spinnshelter includes guyline (or the stuff sack) in the 8.8 oz (tho the line does come with it when you buy it) That'd put the Spinnshelter at 1.9 oz more,or 10.7oz. You' also have to figure a ground sheet into that, at least 1.5 oz for the polycryo one GG sells. You don't necessarily need a ground sheet under a bivy, but you might use one anyways to protect it in certain conditions.
Both manufactures would be able to tell you for sure.
As far as using a bivy, in wet weather, with a single wall shelter, condensation will be an issue over time. So you might end up wanting a bivy anyways if you're taking the Spinnshelter out in the rain overnight. So you might have to figure that into your equation anyways.
On the other hand, if you are looking at extended periods of rain, you might not want to be stuck under a tarp, in a bivy sack hours at a stretch.
I really liked camping under a tarp for the first time. I liked having open views all around me. (It really helps when things go bump in the night; you can look around and seeing it's nothing, or just a deer, rather than wondering whats on the other side of that fragile tent wall.) And the bivy kept my sleeping mat snug in place underneath me.
That's my 2 cents.