A high quality pellet has a higher fibre content than anything else which is ideally what a piggie needs. I don't know what the extra ingredient would be in pellets, I'm not sure there is one. The reason pellets are preferred over mix is that it prevents piggies picking out what they like and leaving the rest so they get all the nutrients from the food instead of just the ones that taste nice. Another reason is that mixes tend to include seeds which pigs don't need and could possibly be a choking hazard.
The best pellet brands are cavy cuisine and science selective, unfortunately you have to be very lucky to find a pet store that stock these, they are available on the internet from sphsupplies.
After that you've got supa burgess guinea excel which is widely available in shops and some vet offices, pets at home nuggets have almost identical ingredient content to supa excell but theyre cheaper. It's all up to you on what you think is best or the amount of money you want to spend. Mine are on the supa excell, they were on pets at home nuggets but I dont have a local pets@home anymore so needed something that was more readily available.
To answer your last question, you don't need to keep them on the same food they've ha for months as long as you change them over gradually to the new food. Mix the two together and decrease thir original food over about 2 weeks or so. Took me ages to get one of my boys to like their new pellets, but he eats them now.
