But which one?
I wonder if they've been ranked.
I'm no expert on Clementi sonatas, but the only one I touched, namely Op. 34 No. 1 in C major, was neither technically simple (1st mvt) nor musically interesting. It comes back to me only in nightmares

Anyway, it discouraged me for getting deeper into Clementi.
Haydn wrote several in D major, if I'm not mistaken. There are a whole bunch of them in the sheet music section of this site. Take a glance at it, and if it's there, please let me know the Hob. number.
The following sonatas by Haydn in the range Hob. XVI/18-52 (which probably includes all "worth to play", i.e. not too early) are in D major: XVI/19, 24, 33, 37, 42, 51.
There is a cute one by Haydn in D maj (forget hob) but it has cheapy bits in it (grace notes). It isnt SO hard and its a great repertoire piece.
This indicates it has to be 37. It's the only one of those listed above with the first theme beginning with some grace-notes. It is a great piece. However, its lively character isn't perhaps distinct enough from that of Pastorale.
I still stand by XVI/32 as a piece contrasting enough.
As for Mozart K.533- I don't know it well, but I was told that the one I've done, K.457, was one of the most difficult, in which case if the other posters are correct this is probably around the same level, in which case I wouldn't want to do it right away. 9 months on a sonata that's way too hard for me feels pointless.
That's interesting, I always thought that K 310 and K 457, the only Mozart sonatas in minor keys (and incidentally the ones I like most), are not the same level, the one in A minor being significantly harder and the one in C minor not being among the hardest ones. However, I haven't tried any of these, so maybe I'm just mistaken. I hope I'll get to K 310 one day. Anyway, working on good interpretation of K 533 has to be a real challenge.