First lens for the NX300

What kind of photography do you normally do?

I think you should figure out what focal length you'd find most useful on a prime lens to help you with your decision. Use your 18-55mm (you wrote 20-55, but I think you meant 18-55) at 20mm, 30mm, and 45mm for many shots at each focal length, and don't allow yourself to zoom. You'll see what it's like to be limited to one focal length. You'll find yourself walking to or from an object frequently and being a lot more careful about framing your object(s).

If you do many people shots at portrait style focal lengths, 45mm will be good for you, or for pet photos at home. 30mm is a more universal focal length where it offers some zoom, but you can still do street photography with it and low light people photography. The 30mm lens is the smallest "pancake" but its focus speed is slow-ish at times. 30mm is my favorite prime focal length for APS-C, and it's the first lens I got after swapping from my Nikon gear. 20mm lens is weaker in resolution than 30mm but is more wide angle - though not really enough to do wide angle shots anyway.

Difficult to summarize in a short post, but while you can do a lot of things creatively with prime lenses, such as do stitched panoramas with 45mm prime and still do people shots with 16mm prime, but generally you'll see that some focal lengths are more preferable for your personal style. Which is why I recommend experimenting with your kit lens first to help you decide.

For what it's worth, the 45mm lens will be a lot more restricting if you go out with it and take no other lenses. Imagine it is like walking around with your kit lens zoomed in almost to the maximum magnification. 30mm will be great for street photography, and I love its field of view for people shots at parties.

Yes, you're right, I meant 18-55mm.

Now that you asked, I noticed I rarely use 40mm and up. I do mostly portrait of my family and pet, using around 30-35mm with some occasional use of 20-18mm when it's a group of people.

The PQ and sharpness of both primes are equivalent? Whichever lens I choose, I plant to bring the kit lens, just in case.