Get the 14. Save thousands. Your weight, you can probably hold it down in 20mph ok. Overpowered is one way (necessary technique, too) to learn. If you stick w/ kiteboarding and find yourself missing too many days due to high wind, THEN buy a second kite. Maybe by then you will want a different brand, year or can just find a better deal.
The same arguement could be made for just a 12, but then you'd have a higher top end. Light wind isn't all that fun anyway (unless surfing.) I learned with just a 12. I'm 185-195 and have 15-20 knt wind typical. There's a bit of a wind shadow here that knocks down the effective minimums for my rig, but I won't take it out below 15knts. I would now in flat water and no wind shadow, but for learning I recommend having enough to slightly too much power.
Big boards are great for learning and get you out on less wind, also. Used boards are much cheaper way to round out your options.
Of course, you've probably already bought by now