If it were just the two of us I'd go for a king bed room onsite.
I'd book now through
AAA with a decent agent (check the resorts board or codes and rates board or I can PM you the person I used) who will rebook you if a special comes out for that time. Make sure you are getting the discounted rate, even if you have to join AAA it's usually worth it.
The reason I'd choose onsite in your case is that you don't really need extra space, and waiting for the busses shouldn't be a problem for 2 adults on a long trip. You can take advantage of the extra magic hours at night, something that a lot of people with young children really can't (like us!). You can also get into the water parks 1 hour before offsite people, a great perk that is rarely mentioned.
I just think that onsite is a lot of fun if you don't have a big family and are planning a relaxing trip. I personally get stressed out waiting for disney transportation when we're in a hurry to be somewhere at a certain time. With a car you have more control. But in May, with no little kids, and lots of time, you can relax and take it as it comes.
Just my background. We've stayed offsite as a couple many times (due to budget), and onsite and offsite as a family several times. I've also stayed onsite & offsite on my own. We don't visit as frequently as some, but I feel like I can compare onsite/offsite accurately.
The meal plan is great, and a good value if you normally eat one sit down meal per day, but if you aren't big eaters and often split meals, it may not be necessary.
Also, you may not need park passes for every day you'll be there. For example you could do 6 park days with hopping and the water parks . The only thing that has to be the same # of nights as your stay is the meal plan.