In terms of upgrading at Envoy, you will have the opportunity to upgrade if you chose to and have the minimum of 950 hours. You'll most likely spend 3-4 months at LGA but definitely be able to get back to ORD especially once the spring flying picks back up. Since you'll be going through an initial training phase you'll also be eligible for the 3 months of hotel at LGA so no need to get a crash pad when you're based out there. Pay for a new FO is I believe between $49-50 per hour. That doesn't include whatever the signing bonus is as well. Envoy is a no brainer especially for ORD.