Although I have never been to the WDW Version of Soarin', I do have experience with the original out at DCA. Now lets start with an admission: I am terrified of heights. There is no rational reasoning for this fear, I have never fallen and been injured, or had any ill effects from being up high. Another oddity is that it is isn't very predictable as to when it will present itself. In the Army, I had no problems sliding down a 90' rope dangleing beneath a helocopter (it was fun, I loved it). However climbing 5' up a ladder has me shakeing like a leaf.
Now, on to my experience with Soarin'. I love airplanes, so of course I wanted to experience an attraction that simulates flight. So when DCA opened I pretty much ran to Soarin' as eager as could be dragging my wife behind. Through the que everything was fine, very interesting stuff until I came bee-boping around the corner and was suddenly face to face with a monster worthy of my nightmares. As my throat tightened, and my heart raced, my morale broke. If I had a tail, it would have been firmly between my legs as I slunk out the door with the riders who had just exited the attraction while my wife (who, by the way, won't even discuss going on anything that even looks like a roller coaster) had a great time seated in the highest row. I was pretty much crushed, but that's the way phobias are I suppose.
Just a few months ago I had a second chance to visit DCA and this time I actually forced myself to ride Soarin'. Of course, God has a keen sense of irony, so I wound up in the highest row of seats. It was awesome. I don't want to downplay the fear; I was downright scared, but the experience was well worth the fear. The sensation of flight is simply unbelieveable. I would reccomend that you try to get seated on the bottom row if you fear heights as I doubt that row gets more that 5 or 6 feet off the loading floor. The top row goes a long way up and you really feel the rise at the beginning of the ride.
So a couple of tidbits of advice:
Do it. it is worth the experience, even if it scares you a bit. (or like me, a lot)
Try for the 3rd/lowest row of seats. (They really aren't that high)
Do it without thinking about it. (Harder to do than say, I know, but find a time when the line is short, and head in.)
And finnaly, If it is really that bad, don't do it. nobody will thnk any less of you. (You couldn't get me on Tower of Terror if I was in a coma.)