I was in the same boat as you a month ago, looking to purchase my first camcorder. I decided that I wanted a Canon and was looking at these exact 2models.
I went with the M30 just because I didn't feel like spending the extra for the S20. That was 3 weeks ago, and this past Saturday I exchanged it for the S20.
I was pretty much overall happy with the M30, except for the so so low light performance. So I realized that was going to bug me, so I decided to fork over the extra money to upgrade. Just in the past 2 days shooting in the same room with the same light as the M30 I notice a substantial difference in the quality in darker areas. Lot less noise.
Maybe the M30 would suit you fine, I just tend to get real picky with things like that and knew that I couldn't stand it.
Plus there are some good additional features on the S20 as well, most of which I haven't learned to use yet.
There is a size difference also, dont know if that matters to you. I personally like it better larger(too easy, dont say anything... haha), feels like a step up quality wise. The larger LCD is cool too.
Good luck and congrats on the baby! If you decide to buy online, there are some good prices on the S20 around. B&H, Amazon.... let us know what you decide and how you like it.