As with bikes, kayak design is a personal preference. I personally prefer the sit-inside type. It's a water sport-and you are bound to get a little wet, but sitting inside tends to keep you a bit drier overall. It also tends to feel as if you are "one with the boat".
Always try before you buy, because they handle differently. Some kayaks are hard to keep in a straight line while others track a little too well and are harder to turn. Ideally, you will find one that does both reasonably well.
1990 Univega Alpina/(stock) gel saddle
2009 Specialized Dolce Elite/BG Lithia saddle
2009 Jamis Coda Sport/Selle Royale Respiro saddle
2010 Jamis Aurora/Jamis Touring Sport saddle