I find herds much more rewarding to keep, I recently added 3 babies into a group and they skipped the most of the hiding, running away, may as well not have guinea pigs in the cage phase, they copied the established adults and have been very confident and settled since arriving. Theres a wider range of behaviours, which I expect is more interesting for them as theyre evolved to live in herds, as well as being more interesting for their humans to watch! It's really nice watching them pick their own friends and watching the dynamics change etc. Having a herd also means buying in bulk, which can actually work out much cheaper!
Cons is an increase in waste, youll have less room in your fridge with all the extra veggies, have to take more trips to buy veg, or youll need to invest in a separate fridge just for the pigs. It will also be more difficult to find who's doing the suspicious poo/wee. Monitoring weight becomes more important to ensure everyones getting their fair share. If illness does strike, there is the risk of it going through the herd, which will be more expensive to treat. Always keep vets bills in mind when creating a herd, should everyone get sick, can you reasonably treat all of them.
At the moment I have a herd of 7, 2 female pairs and a solo boar who lives beside the herd, the only reason I have the 2 pairs is because they can't be integrated into the herd for various reasons. Ultimately though, herds are what I will be sticking with going forward, I just find them so much more engaging and rewarding!