I'm heavier than you (180lbs) and 5'9". I'm very happy on 174. I've never found them unstable at speed, too soft (that famous stormrider problem!) or lacking float (although I've never skied them in more than 18" of fresh). I guess if you are going to be skiing a lot of deep powder then more length might be good (but you'd probably be buying something fatter.
I'm not sure what the extra length would do for me other than give a larger radius. I quite like the radius on the 174 (think 18m) but if you are much keener on big sweeping turns rather than short ones you might like the 184 better.
I'd demo both 174 and 184 and would strongly suggest NOT buying the 184 unless you've demoed it.