I've had a custom Top Gun cover made for both of my MasterCrafts and I would do it again in a heartbeat. I've never had to worry about the cover ripping, never had to worry about torrential downpours drenching the boat, and I have been extremely happy when having to tow with the cover on due to rain. I had dark blue for one, and black for the other. Regardless the color, the boat will get hot inside. I also had two vents sewn in to allow some airflow but never had any issue as our boat was never put away wet.
Our family boat is too big for our garage (I/O), and we had a Top Gun cover made for it when the boat was new in 1991. That cover was just replaced in 2011 due to stitching starting to give up, not the cover. Kept the boat perfectly dry all year long, including winters. Unfortunately, it was replaced with a sunbrella and IMO, Sunbrella is crap unless it's just needed as a dust cover. Don't know anything about the SeaMark style from Sunbrella.
If you truly need it as a cover, for weather elements, I prefer Top Gun. If you just need a dust cover, or occasional use, Sunbrella should be fine.