both are as good has each other. The pellets are compressed with all the goodness inside, same with the muesil, but its dried instead of compressed. I would still give them fresh veg like carrots and the off cuts from cabbage/greens, any off cuts of veg really, but dont give them lettuce, it gives them a stomach upset, and make sure you put out plenty of water.
For young piggies, under a year, look for a good quality pellet with alfalfa. After a year, the amount of alfalfa hay in the pellets should be reduced, as it's high in calcium and calories.
A plain timothy based pellet, supplemented with timothy hay, and fresh fruits and veggies, is your healthiest bet. Most guinea pig mixes contain a lot of "extras" that have a lot of fat and not a lot of nutritional value, which would best be replaced with dark leafy greens, oranges, and the like.
Remember that many guinea pellets have Vitamin C added, which like humans, guinea pigs can not manufacture, so they need in their diet.