I like both names, and both sound great with the siblings. The spelling of Madelyn does look a little more modern (and Benjamin, Emma, and Quentin are all classic older names). If you don't like Madeline, have you considered Madeleine? If you don't like that one either, then Madelyn is fine.
I personally like Lillian a teensy bit better, I think. All the middle names sound lovely. Some questions to help you choose: Is there one you would be MORE sad about never getting to use? Do your other children have long or short middle names? Without knowing the answers to those, I'm immediately drawn to Madelyn May and Lillian Christina. I think I'd avoid Marie -- even though it's an important family middle name, the first names you're choosing are rather common, so I'd tend to want to steer you toward a slightly less common middle name -- especially since you have other important family names that fit the bill.
One thing to notice about Elizabeth: her initials would be M.E.S. (like "mess") or L.E.S. (like "less"). Would either of those things bother you? Neither is a complete deal-breaker, in my book, but it's something to consider.