My bike (a Bacchetta Giro 20) does get more stable with speed. In the picture (assuming it's you), your elbows seem to be out. That can't help. My upper arms are touching my body and don't move much at speed. I steer with my butt once I get moving.
The first time I climbed a hill, I was all over the road and panicked whenever someone passed me. Now I can start on a 10% grade and straighten up pretty quick.
My wife has a long wheelbase, and I believe those are in general easier to ride. I don't have a lot of personal experience, though. I got the SWB so I could put it on my tray style bike rack easily.
It took me about 1000 miles before I really felt like my bike and I were really getting along well.
I agree with other posters.. ride as many bents as you can before buying. Find a place that will allow long test rides (like an hour). I drove 3 hours to find a place.. the 12 hour day was completely worth it. I did that after buying a trike (I didn't even try 2 wheels I was so convinced I wanted a trike) after a relatively short test ride and finding out it really wasn't the best bike for me.