I'd say CURTAINS. But it is apples and oranges. I enjoy both fruits, but sometimes I'm in the mood for an apple and sometimes I'm in the mood for an orange.
CURTAINS is a funny musical comedy, has good performances, and is more of a traditional musical.
COMPANY is a drama, has good performances, and is...well, a Sondheim. So there aren't the traditional "musical numbers" but instead the music is very very very very good. I think the score to COMPANY is better than CURTAINS. However, CURTAINS' score is solid and shines on its own, too.
If you can't choose between a comedy or a drama, flip a coin.
"The Spectacle has, indeed, an emotional attraction of its own, but, of all the parts, it is the least artistic, and connected least with the art of poetry. For the power of Tragedy, we may be sure, is felt even apart from representation and actors. Besides, the production of spectacular effects depends more on the art of the stage machinist than on that of the poet."
--Aristotle