Great question! Guinea pig respiratory systems are so fragile that sawdust is out of the picture, even spritzed I'm afraid. Straw is unfortunately too hard and stalky for piggies, who are at risk of hay pokes. You could, however, use big mounds of soft hay (so not timothy, more like orchard grass?) on top of shavings (pine is okay, as long as it's kiln-dried and almost no oils remain); it also keeps your little ones nice and warm. Hardwood pellets with fleece on top are a pretty good option -- I've seen some people stretch a bag to six weeks or more on guineapigcages -- but are very heavy when wet (I use them in my litter trays).
Use the option that's easiest for you! You can always try something else in future.