This is one of those questions that sounds like it should have a clear right answer, and doesn't. Some children pick up numbers faster because counting shows up everywhere in daily life — stairs, fingers, toys being divided between siblings. Others latch onto letters first because their name, or a favorite word, becomes a personal anchor point.
Numbers are usually a little easier for very young children because counting is concrete — you can count actual objects in front of you, and the number directly maps to something real. Letters are more abstract until they combine into words that mean something, which takes longer to click.
That said, "easier on average" doesn't mean "better for your specific child." If your child is already fascinated by the letters in their own name, following that interest usually works better than redirecting them toward numbers because a general guideline says so.
In practice, most children end up learning both roughly in parallel anyway — counting toys while also recognizing the first letter of their name, for instance. Rather than picking a strict order, it's usually more effective to follow whichever one your child is currently more excited about on a given day, and let the other one develop in the background.
The bigger factor in how quickly either one sticks is how the concept is introduced. A number said out loud while counting real, physical objects — fingers, steps, toys — lands better than a number shown in isolation on a flashcard. The same goes for letters: connecting a letter to a word and a picture ("A — Apple — 🍎") works better than the letter shown alone.
If you want a simple rule of thumb: start with whichever your child already shows curiosity about, keep both moving in short daily sessions, and don't worry about strictly sequencing one before the other. Both will develop, usually faster than expected once the child hits their own moment of interest.
If your child seems to be favoring one over the other right now, that's not a sign something's wrong — it's just where their interest happens to be pointed this week.