The smaller and compass are not really direct competitors.
LESS is a CSS engine. Compass is more of a toolkit around SASS / SCSS and Blueprint CSS framewok. Therefore, if you want to use Blueprint in your project, use Compass.
You should take a look at the LESS syntax and the SCSS syntax and choose the one you prefer. But both of them are now a superset of CSS 3 and have more or less the same features.
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