So I'm definitely not an authority in this area but I did buy an nice 4K TV last year and had to ponder this exact question. I think it boils down to how serious you are about having high quality sound.
Sound bars are a quick and easy way to upgrade the sound on your new TV from bad to decent. They typically do not support the size or spacing of speakers that a real home theater demands.
If you're more serious about achieving quality sound then you need to go the route of AVR + Speakers + Subwoofer. There is a large variety of approaches and costs here but in general you'll be doing more research and paying more dollars and end up with higher quality sound.
I personally built a 3.1 system with entry level Denon AVR, SVS Prime speakers and SVS SB12-NSD subwoofer and to me it sounds incredible. But my dad just asked about this and I recommended he get a sound bar because I knew he wouldn't want to put in as much time or money.