Our first RV was a class A (diesel), and we have a C now.
Some considerations for our family:
-Driveability. With the A, my hubby always had to drive because I wasn't comfortable operating something on a Freightliner chassis. Tried it once, and I think my fellow drivers on the road would agree it's not for me! LOL
-Storage...Are you a large family or traveling for long periods? Make sure you have enough under-body storage for your stuff. Some C's have as much or more than some As, some A's have way more. Check the floor plan and unit carefully and think hard about what you need to pack and carry.
-In that same vein, think about cargo carrying and towing capacity. Do you need to tow a vehicle or trailer? How many people/dogs/suitcases/etc. are going to be on board? I was shocked that many C's AND A's are overloaded EMPTY! During your test drive, drive to a local scale if you can find one. Also, try to find a hill, or at least accelerate during your test drive. We tried a beautiful Fleetwood Class A that had everything we wanted except enough horsepower to get out of its own way!
-Tank sizes...fresh water isn't a huge deal to us as we mostly camp with full hook ups, but if you want to boondock or rough it you need tanks. Gray seems you never have enough! Also, check fuel tank sizes. A's tend to have more, but also lower mpg. Generally speaking.
-Bed quality. Drop that front bed down and lay in it for a while in the FR3. Crawl up and test out the cabover bed in the Sunseeker. I haven't done either, but I can tell you our Class C cabover bed required a lot of "help" before I could stand to even take a nap up there. If you are planning on using all of the sleeping spaces, make sure they are suitable. We have stuffed a 5'10 nephew on the dinette sleeper but I wouldn't make a habit of it.
The Sunseeker has a lot of floorplans, so it's hard to say without knowing which ones you're comparing. We (I, LOL) chose the smallest C that fit our needs because we weren't using the behemoth class A or 5th wheel that we had because my hubby hated to drive them both. We gave up a lot of space and amenities, but for our style of camping (weekend warriors), the C fit our needs best. If I was going on long trips and farther distances, I probably would have leaned towards the A.
Good luck and happy shopping!