The same thing happened with us, Elizabeth...sitting in Premium and getting bumped to First...on the LA flight. But I have a confession: I found the bed uncomfortable and even though stretched out, I hardly slept. And they ran out of time to give us massages. I think I would have preferred staying in Premium, to be honest.
But, I will always choose Virgin over any other airline.
They are the best at handling children as well.
But to answer your question, Stella...in my own opinion, I would opt for the flight that is non-stop over anything else. The last thing you want to do is try to manage a little one, especially when you're pregnant, on and off and on and off a series of planes in different places. Get on one plane, no matter what it is *cough*notLufthansa*cough* and just stay there for the whole journey. It may not have the best service, nor the best meal, nor the most room, but you've only got to endure it, not fight your way through it.
Anyway, it sounds like you've done this by booking Aer Lingus. Let us know how the flight was!