Hmmm......can't really compare the two. Swan is good as a small bit character, but a whole show about her would be too much. Lois Griffin has so many layers to her, that just a smidge would be too little.
As for Alex herself, there are so many options when she's a live person in front of a camera, and she gets to be many characters - the facial expressions are priceless. But again, she's also done more on Family Guy than just Lois - she's frequently many of the extra female characters (TV reporters, neighbors, etc.) on Family Guy.
Tough call, and I'm glad we have the option of both!