well, the tau are very similar to the guard in that they're extremely shooty, but they are far more manouverable and fluid in their attack... (think eldar only slower!)
however, unlike the guard, the tau themselves have absolutely no counter attacking abilities at all!!! (that's why you get the very lightly armoured kroot insteads!
)
the necrons are the army that simply won't stay dead... they can also be a very psychologically dangerous foe with units such as flayed ones and pariahs. they do however rely very heavily on the wargear taken by their lords as none of their squads have sergeant/champion/exarch type options.
though they can play somewhat similar to marines, i'd go for necrons as they are very, very different in both style and abilities from the guard!
cheers!