Greystone 30°

Big Agnes generally has better quality control in my experience, but honestly both of those bags are solid choices for summer camping. The Bishop Pass might pack down a bit smaller if that matters to you Have you considered the REI Co-op Magma 30? It's in the same price range and gets way better reviews than either of those

Spec-wise they're pretty close: Greystone 30 is a value down bag (600-ish fill, 1 lb 13 oz range) and the Bishop Pass 30 is a tiny bit lighter with slightly higher fill (650 duck down) and a legit EN comfort around 40F. In real life that means the Bishop Pass is usually a safer bet if you care about warmth-to-weight and pack size.

With used stuff it can be a little harder to see year to year differences. If you can find specs for the EN rating and you are a cold sleeper gonfor the comfort rating. If you are a warm sleeper go for tge lower limit. Those are all probably good mid range brands though can veer more fashion at times. I like Marmot bags bit the Col looks pretty gimmicky. The rating for the Big Agnes brand shows it to be nominally rated between the lower limit(warm sleeper comfort) and extreme(survival) rating. That kinda puts it in Coleman class but for me would probably be plenty warm in your described conditions. The 600 fill down though is going to be a lot bulkier and heavier than the 800 fill power stuff in the other two bags. 

I went rei big Agnes, but I also have 2 (20 year old) down bags that I put inside each other that can keep me just as warm and combined they are as small and light as big Agnes but with down moisture is the enemy and in winter there is plenty so there is that

Check out the REI outlet online. The last bag I got there was from lightweight Big Agnes, and the discount was 40%. Great backpacking bag so far!

I have a Big Agnes single I LOVE so I’m jealous of all the folks here with the double. We have a flatlands fleece double bag and the Klymit double pad because they were the easy buttons through our wedding registry. The bag packs large for my taste but the pad is perfect. We sleep on top of each other at home and love snuggling all the same in the wild. Unless your trips are all ultralight backpacking, it’s quite cozy and romantic!

I would go with a quilt, they are great for moving around. Zenbivy is most peoples favorite, I really enjoy my Big Agnes.