What’s best for attic insulation
Blown in insulation fills the cracks better. For that reason, as a second layer it has big advantages.

Downside: You have to borrow/rent the blower, which, if you are in my situation, you need to plan two trips to the hardware store an hour away.

Upside: Blown insulation goes in very fast. My stepson and I did one section of my house, 28 x 40 feet in about 1.5 hours, once we were set up.

Downside: This is a really dirty job. You need to wear goggles and a good dust mask. The coffee filter cones aren't adequate for this. It also gets in your clothes, hair, ears...

Downside: It takes two people: One to break bundles and put them in the blower, one to crawl around in the attic.

Batts: My experiences with batts in attic spaces has been dismal. Lots of places where gaps were left either because of blocks nailed to trusses, or batts that were sloppily installed. Batts are for walls.

You have a choice of either fiberglass blown in, or cellulose. The latter is in essence finely shredded newsprint treated with a fire retardant. It's R value tends to be lower than fiberglass, but it's also substantially cheaper. Also is more sustainable.