Both times on the JMT I started in late July, but even starting earlier in the month (and especially this year) I suspect a 20-degree bag would be perfectly fine. It will get down below freezing at night, but you can share some body heat.
A tent or tarp will also help you conserve warmth.
As for the bags you linked to, they all look like good choices. I cannot personally comment on any of them (I haven’t tried them) but I like Nemo as a brand and the gear review site gave a good breakdown of choices.
Ultimately, you’ll pick one and then try it out and see how it works out. Sometimes you can borrow gear from friends or rent gear to check it out (try REI). Let me know what you end up getting!