More views, at :

http://www.onestopenglish.com/support/ask-the-experts/grammar-questions/grammar-two-syllable-comparatives/146356.article

There are no rules! Only tendencies. (In fact, you could probably say that about ALL grammar). For example, a quick search of corpus sites on the Internet threw up the following 'exceptions' to the rule that one syllable words take –er in the comparative:

As the sun grew more fierce, our complexions darkened.
she was more full of good intentions than ever
she said if you had something like porridge, you'd feel more full
A less religious or a more sane man than the fanatic would have shut up.
You'd think it would make them more sane and agreeable than the norm.
it seems to me more brave to stay on Earth and explore inner man
To swim is true, and to sink is true. One is not more true than the other.
somebody came up and told me which is even more rude.
could any argument be more tired?

Some one-syllable adjectives, like right, wrong, and real, virtually never take –er. (Is that a rule or a tendency?).

The Longman Grammar of Spoken and Written English, which is based on corpus data, suggests that forms like more fierce are more emphatic (emphaticker?) than fiercer. They go on to say that two-syllable adjectives show even greater variability. The tendency, however, is to inflect (i.e. add –er, -est to) adjectives ending in –y (pretty, prettier, prettiest) but adjectives ending in –ly can go both ways (lonelier, more lonely). Sometimes inflected are adjectives ending in –ow, like narrow, and –er, like clever, and –le, like simple.

Those that usually take more/most are adjectives ending in -ful, -less, -al, -ive, -ous (most useful, more mindless, etc) and adjectives formed from participles (more bored, most tiring). And of course, three-or-more-syllable adjectives. But despite these 'rules' the following instances have been recorded: unhappiest, boringest, and raggediest. As I said before: no rules, just tendencies. Can I be more clear?