I downloaded HxH 1999 and will watch it tomorrow as I'm watching another anime. I've never watched anything HxH related but I have watched and enjoyed other Shounen jump anime.
What should I expect?
You should go with 2011.
Watching 99 will turn you into an insufferable shitposter in future HxH threads.
99's has better drawing and animation in general (until Greed Island). 2011's pacing is much better and don't add unnecessary shitty filler.
Read the manga instead.