Lasius claviger is the slightly smaller of the two at about 3 or 4mm long. They're a brighter orange yellow color and their eyes are relatively small in comparison to their head.
Both species live almost entirely underground though I've noticed L. claviger sticks with soil nests wherever there are the roots to trees and large shrubs. While L. umbratus more often nest in mixed nests of rotting wood and soil, and I'd associate them more living along side termites.
While all this is well and good, I've noticed there are perhaps 4 species in yard. What I'm calling L. umbratus is at least one species here, though maybe not L. umbratus itself, certainly something in the umbratus group. What I'm calling L. claviger however seems to have at least 3 forms. L. interjectus I'm sure is one and differs only in that the hairs along the gaster are more parted than the L. claviger. The third version is almost completely yellow, smaller looking, and more easily stirred into producing the citronella odor. I didn't encounter this one today but if I find them again I'll try and get pictures, key it out, and report back. They seem to be completely devoted to wet muddy clay soil though as I've never seen them when it's dry out.