Pardon the lengthy post, but you might find some of this helpful. I get out at least two days a week and sometimes three. Even now during cold weather I’m sailing just as much as I did this summer. Great fun and I spend a lot of time experimenting with various aspects of the Hobie products.
I bought an Adventure Island last year. Very shortly afterwards, I bought a Tandem Island. Both boats are 2012 models and feature roughly the same design and assembly.
I rarely use the AI any longer. Great boat and other than the tub of water you sit in (no problem in the warm months) I have zero complaints about it. The TI, however, is just as easy to sail solo and offers some big advantages, as some have pointed out here, in the area of payload capacity, sailing speed and the ability to take another person with you.
There is a common complaint that I’d like to address, as I think it turns out to be far less of a problem with the boat and more a problem with the user. The first time I took my TI out solo, the wind was exceptionally brisk. Gusting to about 35 MPH and steady to at least 20 MPH. Sitting in the rear seat, I launched and before I could get underway with the Mirage Drive the wind pushed the bow downwind. Normally I’d just start pedaling and push the rudder over and quickly come about. Not this time, however. Nothing I could do would bring the boat around. I struggled with it for about 15 minutes and decided that I’d have to make landing downwind and then figure out how to either get the boat turned back into the wind, or get a vehicle down there to retrieve it.
As it turns out, I was doing two things wrong and once I fixed those two things, the boat behaved and pedaled much more like the AI. First, the longer and higher hull offers the wind so much more force to act upon that trying to steer the boat in heavy wind without having the centerboard deployed is almost impossible. But drop the board and suddenly the steering characteristics change completely. Now the board helps to prevent sideslip in heavy wind as the wind doesn't find it nearly as easy to just push the bow around.
The next thing regards trim. In the AI most of the weight is just aft of the boat centerline. In the TI sailed solo from the rear seat, most of the weight falls well aft of the boat centerline. This is exacerbated if you have much gear in the tankwell. The nose rides high, offering the wind more surface to act on and pretty soon the rudder is unresponsive and pedaling requires far more effort.
Since I’ve begun deploying the centerboard 100% of the time (provided I have the depth to do it) and moving some gear to the forward seat for better overall trim, I find that my TI paddles and pedals on par with the AI. I don’t feel I’m giving up anything and I have the ability to haul more gear, under more sail (and more speed) and carry a passenger if needed.
The only nod I’d give to the AI, which I still enjoy although seldom use, would be in a situation where I might have to man-haul the boat over shallow mud flats or across obstructions, paddle for many miles without the aid of the Mirage Drive, or didn’t have a boat ramp to launch from. This is where the lighter weight of the AI would be a plus, and I as understand it, where some of the Watertribe courses might favor it over the TI.
Otherwise, once on the water, a little extra weight is of no consequence and can be beneficial to some degree. I find no difference in the durability and reliability of the two and if I was limited to just one, I’d buy the TI.
Because these are fairly expensive for “kayaks” (not very expensive for sailboats, however) I’d recommend visiting a dealer that has both available for demo and spend an hour in each. They’re both exceptional values considering all the things they’ll do and you’d be hard to pressed to go wrong with either.