For a start Chinese Hamster's aren't dwarfs. They're in a different genus, that's why they live alone, and why they look so different to the other species. I've had quite a bit of experience with all of the dwarf species, and Chinese Hamsters.
Winter White/Russian Dwarf:
If you're getting these from a rescue or a breeder chances are they'll be lovely, large pet stores tend to get their animals from mass breeders who aren't intelligent enough to tell the difference between the two, so get hybrids that are often very aggressive. Make sure you can handle the animal before buying one of these two species, if you can coax them into a tube and then your hand without them trying to bite, they'll be perfectly easy to tame. Campbell's Russian males tend to be a little more bad tempered, but are tamable if you've got the patience. The females, and both sexes of winter white, tend to be very laid back, and will rarely bite. Males usually tend to be more active. Neither male or female tend to fight any more with their own kind, so they're pretty safe in groups so long as they were living together when you bought them. I currently own a male and female breeding pair or Russian Dwarfs, expecting their first litter soon. There's little difference in regular appearance between the two, so make sure the person you're buying from knows their stuff. Winter Whites will usually have a much darker stripe down the back, and dark arches down their sides, as well as lighter eyebrows. Both species can be grey, but only Campbell's Russians are often found in shades of brown.
Roborovski Dwarf:
These are often seen as impossible to tame, but they really aren't, you just need to use the right taming method. I've had experience with 2 of these, and I managed to get one out of the cage, sat peacefully on my hand within about 10 minutes. It differs from hamster to hamster though, they are very nervous and will bolt at light speed if scared, so you need to be careful during taming to avoid injury. Also very small, so not great for younger kids who might damage their delicate little bodies. They're the most social hamster and actually prefer to live in groups. Again, they'd rather run away than bite. Because they're so small they don't really like to climb as much as other hamsters do, so prefer one large space to run around in to a multi level cage.
Chinese Hamsters: Chinese Hamsters in my opinion are the friendliest species of hamster you'll find, and I'd recommend them to a first time owner over a Syrian any day. Although fast, and initially quite nervous, after a few days most of them will be happily running up and down your arm, sniffing your ear or sleeping in your hood. They're brilliant climbers, and it can be quite hard to put the down when they want to play, they'll cling onto your fingers for dear life. And it's so cute when they curl their little tails around your finger so they can stretch over and have a sniff of something. I currently own 2 females, who should always live in their own cage. I've heard of people who've managed to keep boys in pairs, but it's not really advised, they're solitary animals, like Syrians. Although boys might accept another, it's not really worth the risk.
Overall I'd recommend you go for one of those Chinese Hamsters the shelter has available, and if you're not very experienced with dwarf species then I'd say second choice is either Winter White or Campbell's Russian.