What are the similarities and differences between SNAP-tag and CLIP-tag?

SNAP-tag and CLIP tag are two self-labeling proteins derived from human O6-alkylguanine-DNA-alkyltransferase. Both are monomers and both are extrinsically fluorescent proteins. Both have a length of 182 amino acids and a molecular weight of 19.4 kDa. 

CLIP-tag is a modified version of SNAP-tag. The only point of difference between the two is that SNAP-tag recognizes and reacts covalently with O6-benzylguanine derivatives while CLIP-tag recognizes and reacts covalently with benzylcytosine derivatives.