We have an inflatable dinghy and motor, and a couple kayaks (one sit-in, one pedal Hobie sit-on). We don't really have the room on our boat for the kayaks AND the dinghy. We chose the kayaks this summer. This is just one couple's perspective: lighter is better. My wife has a light weight sit-in, and either of us can heft it to the top; my 53 pound Hobie takes both of us to get it on top. I have no issue putting the inflatable dinghy on top by myself, and it weighs about the same as the Hobie... just spreads the weight out easier and it doesn't scratch anything as you move it around.
The kayaks are great fun. The inflatable dinghy is easier to get in and out of from the boat. The inflatable is a good "delivery vehicle", perfect for carrying groceries, laundry, several people, and a cat. The kayaks are... great fun. If you have to get from a lumpy anchorage to go ashore to get a weeks worth of groceries, a dinghy is the answer.
We have owned an Advance Elements tandem kayak, and both prefer the hard shells (singles)... they ride strapped on top of the boat when we are towing (or while underway). I find the Hobie pedal kayak comfortable, and easy for a reasonably out of shape guy to go all day (it's like walking... well, using a stair stepper). My wife definitely prefers the paddle type sit-in she has. One drawback of the Hobie is it requires additional storage (pedals and seat)... I find that a small price to pay for using the Hobie. I also have a sail kit for it - even more fun on the right day.
So, all this is saying: a kayak as a tender can work, but it isn't going to have the carrying capacity of a dinghy and will likely not be as dry. Depending on the inflatable kayak, paddling solo in a tandem kayak can make for some balance issues. If you are in warm, generally calm water, you can make most things work. I find the dinghy a better all-around to-and-from shore vehicle than a kayak. But, did I mention that the kayaks are fun? 😉
Good luck with the decisions and congrats on the new boats mentioned in this thread!
Best wishes,
Jim B.