For ultralight fishing, a spinning reel is best. These reels are generally lighter (depending on the size of the reel) and have a more concealable line, which is ideal for freshwater and saltwater fishing alike.
They also have a faster retrieval rate, so anglers often prefer them as they can catch more fish in a session. Baitcast reels are great for regular fishing (learn about the champion-named Abu Garcia Jordan Lee Baitcaster Combo here) as they are heavier, which provides better casting accuracy and distance. However, this weight can be redundant in ultralight fishing. The following post regarding the parts of a baitcasting reel and the best baitcasting reel for bass provide more great information for your reference.