Great feedback, all! Thanks!
I think I'll play with the second dress a little and see if I'm moved to wear it in the next week, since it's now getting hotter here. I think the vent-and-tie treatment goes along way to break up that large swath of patterned skirt and create a more flattering line -- it sure helps with the solid blue Anthro dress as well.
The downside about the first dress as a wedding option is that I think it needs black shoes and I want to wear the Rockstuds for the wedding. I do think this dress would work well with my black LD Tuttle sandals -- if not for the wedding, then for other occasions that fall somewhere in the dressy-casual continuum.
The fabric on the first dress (Tahari if anyone is interested) is that slightly stretchy, substantial neoprene-like fabric -- very similar to the floral dress I was planning on wearing to the wedding. Not the most breathable fabric in the world, but comfortable for movement and very forgiving of the shape underneath. So it is unlined, and I think it would handle the slit alteration quite well.
Thanks for all the responses! Will ponder further...