Human nail clippers are shaped to human nails, so can pull on the guinea pigs nails and cause them pain if you don't clip at a specific angle. The round ones are round to fit the round shape of animal nails.
I find it best to get your clippers from a vet, they have sharper more professional ones. The sharper the clippers, the less chance of snagging and pulling, and half-clipping by mistake. If you ask your vet to order some in for you, and you'll pay up front, they're usually fine with that.
I personally get my vet to do it for me, because I have no one to help me to it and it's a two person job with my squirmy girls. I do it every 2-3 months usually, or sooner if I feel like they're getting scratchy. It's tough with my girls though, their quick goes almost all the way to the tip. I tried frequent tiny trimmings to get the quick to recede, but it went no where.