Here is our set up, Me, wife, girl 11, girl 13,
We kayak exclujsivley on Ozark rivers and streams. I guess I bought the kids their own yaks when they were about 10, they couldnt handle their own before that. I have a tandem sit on that I carried the kids on until they got their own. Ill probably get rid of the tandem this year and get a solo fishing yak.
We prefer the sit on, I think they feel more stable, and its easy to roll on,roll off to take a dip, they dont fill with water, so no bailing. They are heavy, but my Dodge 3500 dont care. I usually haul two on top, and 2 in teh back, except for long trips, then I take the trouble to mount 4 on top. Quite a site with yaks on top, and 4 bikes mounted on the front of the truck.
The shorter ones are more maneuveravble on the swift streams, the longer ones track better in slow water or lakes. You dont have to spend $700, unless you are going to get realy serious. I got an 8 foot sit on top on sale for$100 at Dicks last year, (just after I bought the same one for $199 about 6 months before) so watch for sales at Dicks...