The Prism is a nice boat, but I have only paddled it with a light load.  I agree with the other solo paddlers, frequent trim adjustments based on the wind and yes, tailwinds are much more challenging.
I paddle a Merlin.  For a number of reasons, not choice, I had to remove the seat.  I had a sliding bench seat glassed in.  I love it.  With the footbrace in she generally handles beautifully.  I have often been the only paddler on the water, with glum paddlers campsite bound, staring at the waves.  
So far as paddling a straight line or being able to stay on one side:  I hit and switch, how often depends on which quarter the wind is coming from.  If the wind and trim are right, I can paddle on one side for a very long time.  I'm used to traveling in a sinuous line.  
One of the reasons I like the Merlin is that, unlike it's slightly longer cousin, the Magic, I can lay my packs down flat (unless I seriously over pack).  So far as capacity goes, I'm a big guy (230#) and I have (over)packed it for a 20 day solo and been perfectly happy with the results.
I love a dedicated solo boat.  Someone else mentioned the Wilderness, which I like, too.  If you want something with a little more rocker (cuz, you might need a dedicated river boat, as well as one for the lake country...) check out the Argosy.  Fun boat.  
Nuff said.  Good luck and have fun.