To answer your question, here it is.
EWR 737 FO - I get 16 days off a month, however not always commutable on both ends and I'm at 50%. If you are a new hire you will get 12 days off and not commutable on either side usually.
EWR 757 FO - Gets 12 days off, unless you are in the top 10%, then you get 14 days off. Almost entirely commutable due to Atlantic crossings, 7 a month or more. Some guys will work 15 days in a row and then get 15 off, but that's a little too long away from home for me.
EWR 777 FO - The most junior lineholder gets 18 days off a month. All trips are commutable. The more senior people get 21 days off a month.
As far as IAH goes, it's pretty much the same as EWR, but you will have better seniority in EWR. I've only been there 1 1/2 years and moving up rapidly.
So to answer your question, most FO's are staying put on the 737 or they are trying to get the 777 just for days off. I missed the 777 by one number on our last bid
Cheers, and I hope that this info helps you out.