This is only my opinion based on using all three of these products: Fray Check can leave crusty areas that have been 'sealed' for fraying, and I have found sometimes the color of the fabric changes slightly. Fray Block dries softer and clear. I prefer this over Fray Check. Clear fingernail polish can dry hard and crusty and does not disappear or blend into the fabric.
Fray Block comes in a tube with a nozzle. Use a very small pin to poke the hole opening so too much is not coming out of the tube at once. I get it a Hancock Fabrics using coupons.