Megan,
I think you’d be well served by either the NEX 5N, 6, 7, a5000, or a6000. These APS-C sensors offer some of the best image quality you can find. For long exposures, especially night shots m4/3 is going to show its weakness, otherwise differences in IQ are probably not worth worrying about much.
Fuji and Olympus are where you want to be looking if you’re leaning toward m4/3 at the end of the day.
A good friend of mine switched from a full frame Pentax DSLR to the Oly E-M5 and has been mostly happy with it but he likes to shoot star trails and other long exposure shots and I know he’s been a little frustrated with excessive hot pixels and noise. Granted that sensor is starting to show its age now, but I think comparing modern APS-C to modern m4/3 it’s generally going to be the case.
Here are a couple of older blog posts that might be of some use to you…
http://hikeitlikeit.com/2012/photography-and-backpacking-8/
http://hikeitlikeit.com/2013/olympus-omd-em5-impressions/
I still find my 5N to be a great camera. I recently purchased an RX100 to take on runs and it’s pretty impressive too… but it can’t hold highlight or shadow detail like the 5N can. I’ll be posting a writeup about it soon, as well as some thoughts on the full frame mirrorless and why I’m not switching to them (as if anyone really cared :)
TL;DR… 5N can be had for a song. Just sayin!
