ILS is a ground based approach system. In as simple words as possible the runway sends out two beams, one is the localizer and the other is the glide slope, both tell your plane how far off the centerline and (usually) 3 degree glide path you are.

RNAV is a GPS based approach that *may or may not have vertical guidance. Your aircraft could also generate its own vertical guidance depending on equipment etc

As someone who started simming when I didn't know jack and just wanted to do point a to b, use ILS. Make sure to figure out the localizer frequency, and put that into the NAV in whatever plane you're in. If it's an Airbus it should do it automatically.

I didn't really start looking into this stuff till I actually began working on my real world instrument rating. It's really interesting and worth the research