Alfalfa based pellets are better for rabbits and guinea pigs, up to one year of age and breeding does and sows, because it has the necessary calcium that young ones need for growing strong healthy bones and eyes, etc and that breeding sows and does need for the pregnancy. Once they reach the age of 1 though, it is advised to switch over to a timothy based pellet, because of the increased risk of kidney stones and bladder stones.
I have 3 guineas who are on a timothy based diet and get alfalfa/timothy cubes to chew on, along with their timothy hay. I also did the same with my rabbits, when I had them.