Beide Formen gehen; es kann vomm Satz abhaengen.

This engine is much quieter than the other one.
This engine is definitely more quiet than what I'm used to,

e.g.

??
Wo ist denn der Unterschied?

Da ist keiner. Sag' ich ja, beides geht.

The Chicago Manual of Style recommends:

A comparative adjective expresses the relationship between two things in terms of a specified quality they share, often to determine which has more or less of that quality (a cheaper ticket, a happier ending). The suffix -er usually signals the comparative form of a common adjective having one or sometimes two syllables (light/lighter, merry/merrier). An adjective with three or more syllables takes "more" instead of a suffix to form the comparative (intelligent/more intelligent, purposeful/more purposeful). Some adjectives with two syllables take the -er suffix (lazy/lazier, narrow/narrower), but most take more (more hostile, more careless).

A participle used as an adjective requires "more" or "most" before the participle; no suffix is added to form the comparative or the superlative (this teleplay is more boring than the first one", "I am most tired on Fridays").

For what it's worth, I say "quieter" and "quietest".
(Yes, dude, I do hear people say "more quiet" but I don't say it. *g*)

Oh boy, here we go again. :-)

To me, it's more a question of rhythm and how the sentence flows, I suppose, especially a written sentence as opposed to a spoken one, although euphonic speech is certainly attractive, too.