The Parliament just feels like a more beautiful version of the Nebula - it is rather elegant while keeping that Galaxy-class kitbash feel, combined with a bit of Miranda in the square edges of the saucer and a smidge of Sovereign in the nacelles. Nebula just looks derpy by comparison
True, Nebula and Parliament have a bit different purposes. However, other than nacelles, they actually seem to be about the same size interior-wise (based on some very Memory Beta info).
The deflector on the nebula class just always looks like a dumb little mouth to to me