ooooo. I can't choose between those three! But I can tell you my experience with them
guinea pig - these guys are soooooo sweet. I love them! Mine never once tried to bite and I love the noises they make. They will need a much larger home than gerbils or hamsters and love raw veggies.
hamster - The larger hamsters need to be kept by themselves. The dwarf hamsters can be kept in pairs. I like both, but only kept the larger Chinese hamster. Hamsters in my experience are more prone to nip and it can hurt. They need to be handled every day to keep them "tame." They are less active/jumpy than gerbils and are also less skittish if handled every day.
gerbils - I LOVE my two little girls I have now. Girls need to be kept in pairs and boys need to be in groups of two or more. Never keep just one gerbil as they need to social activity (grooming and other interactions) from other gerbils. I have never been bitten by my gerbils, though one of them does like to taste me. They are very active and I have created a playpen for them to run around in. A wheel isn't enough exercise for these critters. If the space is large enough, you'll see just how fast they can get. Unlike hamsters, gerbils don't need to be handled every day to make sure they remain "tame." However, they are not cuddly animals. I find them very fun to watch and handle though. Also, they smell less than hamsters or GPs. Actually, I've never detected a smell from my gerbils. Oh, and two gerbils will be fine in a 10 gallon tank so long as they get to go to a playpen, but as always, the larger the home is the better. You can find some need top extensions for tanks so that they have many layers to play on.
If I had it my way, I'd have all three and them some other creatures as well.
I do suggest you get them from a breeder. I found a guy who flew my gerbils to me for free because it was just a hobby for him. You can google search for different organizations that will list breeders near you. I used the American Gerbil Society to find my gerbils.
Good luck and enjoy!
PS-check out Degus as well. I know Menagerie has a pair. These are great creatures too.
*edit* and one more thing (sorry this is so long!), pleeeaase don't ever get a wheel that is just metal bars. legs and tails can get broken in those. I am sure you already knew this though. But I just wanted to make sure.