I did the same thing years ago. You'll have an edge since you've been riding waves, so ride whatever you think you'll like more. If you start on a longboard, you'll get a lot more waves and start figuring things out a lot easier and quicker, then you can transition into smaller boards. Plus, longboarding's really fun, don't rob yourself. We get a lot of knee high days in New England that can be a blast on a longboard! Get a decent board though, something 9' or bigger, and something that will keep you interested after you learn. Most beginner boards are slugs and you'll outgrow it quickly. Have fun and good luck!
hello fellow noob,
i'm 6 ft 210 pounds and had a couple lessons on an 8 foot soft top board and found it easy to catch waves, pop up, and ride down the line. now i've got a 7'8" funshape fiberglass board and its fairly easy to catch waves but much harder to pop up on as it is much less stable and i need much more practice. i wouldn't go any shorter for now so that's my current experience.