I really like Lato. It was the typeface I used for headings on this site. That was until I fancied a change and discovered Calluna and Calluna Sans.
I prefer straightness of the leg on Calluna Sans' uppercase R, and the way the uppercase J drops below the baseline. The uppercase M nearly put me off: I usually prefer vertical stems to sloped, but the apex (in the middle) is cleaner than that belonging to Lato, and overall it appears less imposing.