What to do?
My criteria for a second bike was...nimble, quick, low (I have the opposite problem you have...I'm a short $h!t), fairly lightweight, fuel injection, tubeless tires, able to be set up to go cross-country if I wanted, and gets decent fuel economy. My '07 Sportster 1200 Low fits that bill. Not a whole lot of bikes manufactured fit that bill for me.
The Vulcan is cheaper, more forgiving, is capable of long range riding (though it won't rocket UPHILL in the mountains), gets good fuel economy, takes regular unleaded, if I'm not mistaken, standard 10W40 motor oil, and is comfortable from the start. It will not beat a 1200 Sporty assuming same rider skill. The problem with a 900 is it's still a 900cc and as your skills improve, you might want something with a little more "oomph", especially while merging into traffic and passing. Then you might want another bike you won't be able to afford for a long time. So I've probably just helped increase your dilemma even more.
On one hand, Kawasaki Vulcan will WORK and it's affordable to you (so will the Honda 1100, 1300, Stars (Yamaha), and Suzuki. The Triumph American might also be a consideration, but I'm not sure how affordable or reliable they are. However, I'm not sure if those bikes will perform how you want them to perform once your skills improve. You don't appear to be able to simply "get a starter bike" and then simply dump it for a better one. You seem to want a bike you can learn on and keep riding happily once you have some miles under your belt.
On the other hand, you want a Harley, and I don't blame you. There's a certain feeling one gets on a rumbling 45 degree V-twin, but the Sportsters have fit and comfort issues for you in stock configuration, and the Dynas seem to be a little out of your budget right now. I'd say if you could afford it, get a good used Dyna.
As an aside, but related, the most common complaints I've noticed from ANY make bike is the seat and shocks. And I've noticed that although Harley accessories are expensive, they are reasonably priced compared to other bike makers accessories, and Harley has more accessories to choose from. And I LOVE the ease of installation of Harley seats....one bolt! Other seat manufacturers also make plenty of seat choices for Sportsters.
So, bottom line in this long-winded, rambling post? If a Sportster 1200 with forward controls is within your budget, and you can swing up to $300 for a seat (I doubt you will need to spend that much), then get the Sporty.
I have 11" Progressive 412 shocks on the rear of mine, an after market HD seat, and I've been close to 19,000 miles on it since September 2007. It still works for me.
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I've bought close to 60% of all my aftermarket parts from members here. No complaints and it fits my budget. Just an idea.
(added bonus) I know for a fact they will let you test ride the sporty if you have an endorsement on your license, but I don't know about the yamaha, it wouldn't hurt to ask. I would ride both if possible and see which you like. Keep in mind, if you ride the vulcan with friend with harleys, you're going to be left sitting from time to time. Even the 883 is considerably faster than the 900.
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