Mannan binfing lectin recognizes sugars residues like mannose encountered in the surface of polysaccharides like the ones presents in the LPS of Gram-negative bacterias. My question is if the lectin pathway will be activating complement in case of finding LPS or will it be the alternative pathway, wich recognizes LPS too.

This is just my gross understanding, but it would be the lectin pathway. Realistically, as activation of the lectin pathway (as well as the classical pathway, for that matter) can, in turn, activate the alternative pathway, they'd eventually be going simultaneously against the same pathogen.

 

With the alternative pathway, C3b/C3i has to be created before the pathway can be activated, either by spontaneous degradation of C3 by water, or by one of the other two complement pathways. So alternative would come after lectin, but, again, nothing prevents them from going at the same time.