Hi,
both of the mentioned ones, the moonlight 3rd movement and the Waldstein 1st movement, are difficult.
My personal opinion is: The Waldstein 1st movement is more "convenient" to the hands. But we must be very careful here. Both of the mentioned movements demand thorough and good CONCEPTS, for playing them.-
Both of the movements might contain (but that's only my subjective view) elements of "dialogue", which should be brought out well.
But the Waldstein 1st movement has additional features the moonlight 3rd movement cannot provide.
For example, there's a very demanding (short) syncopated section in it, which is, then, near the end of the 1st movement, followed by demanding left-hand-actions, to bring out the relevant voice.
But even before that section, there are wavelike structures, in which single upper- or "treble"-notes have to be emphasized... .
I would - personally and subjectively - say: Make up a fitting concept in your brain, first, IF Waldstein 1st movement doesn't seem to be too difficult for you.
I don't know how far you already have gone in studying the pieces, we're on an online-forum here, so I can only say the following:
I would never discourage ANYONE to look at the Waldstein-Sonata. But beginners or people who might be not too experienced yet in Beethoven-"PATTERNS" and "features", should be aware that this sonata ( and the moonlight, too ) are very demanding, and "dangerous".

The "Concepts" which one could make up for the Waldstein-Sonata may contain (subjectively):
Waves of the ocean, beginning small, then getting bigger. Then accumulating. Sunrise. Beginning. Dialogue. (Dialogue is imho very important in movement 3 of the Waldstein, too: "Question" and "answer").
Cordially, 8_octaves.