CTX series
With a 5 month old, I assume that your time is somewhat limited? The CTX series is great for making sure you get in all the weight training and cardio in a shorter period of time. If you find that it's too hard on your joints doing all 5 cardio portions ( as I did ) you can substitute other cardio ( like 1 or 2 sections from Step Heat )for some of the sections. You can also combine the weights if you find the 1 part per day to be not as effective for you.
The PS tapes are great because they have the weight training split up over 3 days. You could toss in a 20 minute cardio section on the upper body days really easily, so you keep your cardio momentum going. I had the greatest physical changes when I started doing the PS tapes, but keep in mind I had already been weight training for acopuple of years. I can't really comment on how they would affect a beginner to weights.
MIS is a great total body workout, but at 75 minutes, it gets pretty long, and can be overwhelming at times. What I do is stop right before the abs, and do those on another day. That way it's just 60 minutes plus a little more stretching on my own. Sicne I only work my abs 3 or 4 days a week,I can easily get in what I need on other days.
I hope this is helpful. Good luck!!
Erin