Yes, The number of sts that you're getting per inch will tell you which way to go in needle size. Take a look at the videos on gauge under the Free Videos tab at the top of this page, Getting Started. There's lots of good advice on gauge which is so important.
Number of plies is an unreliable measure of yarn size and weight so go with the number system or the gauge given in the pattern in sts/inch.
Also, when you measure gauge, be sure to knit a good sized swatch, about 5"x5" or 6"x6". That way you can more accurately measure over the middle 4" and estimate your sts/inch.