Then here's what you need to know: you are standing farther forward on twin tips, so on a 150 twin tip you have a lot less ski in front of you than on a 150 regular ski. Also, the better you get, the longer you will want your skis. When people say they like "shorter skis" for park they don't mean skis shorter than their chin or something, they just mean they don't want like 190cm powder skis or something. Measure from the floor to your chin and then from the floor to the top of your head (i'm gessing that for you this will be around 165cm to 175cm). This is a good range to be in, then adjust it for how you want to ski or your ability. If you get 150's, you are going to get tired of them really fast and everyone will make fun of you because you will look like you are on snowblades or child's skis.