We have chartered in the Carrib with both of them over the years. In some places, the offices now are side by side in the same marina , and I am told have the same Manager for both offices. Someone with sunsail told us that Moorings has a "different target audience", (my words, whatever that means.
I would think that in a marina where they both are located that they may share dingys, and boat mechanics too. My only moorings exoeriences (all good) were 20 years ago, so hard to compare, recent ones are with sunsail and were at different marinas, but moorings then seemed more helpful as in welcome!, we will get a luggage cart and help you get to the boat, sunsail perhaps less so.
Anyone have an opinion, experience?

They are both owned TUI. They have similar guaranteed income programs and use different boats. I would think all maintenance is done by TUI employees.

As Evan said, they are basically the same. There's only a few differences.

Each one has a few unique base locations. Moorings boats tend to have shore power A/C where fewer Sunsail boats do (at least where mono's are concerned) and pricing is handled a little differently as are start times.