Expect pay to be lower than you're used to, unless you're coming from the rural South.
I think there isn't a bad place to work in the Lou unless you're looking at Kindred or Select, which don't apply to you.
BJC gives expert pay in some areas (ED, ICU, periop) but I'm not sure if it applies to GI/IR/cath lab. Expert pay = more $ to nurses with more than 2 years of experience in those areas. BJC does NOT start health benefits at day 1 of hire, which can be a negative. SSM and Mercy both have a good benefits package. Barnes is a great place to work if you do not mind the commute and the parking issues. You need to use Metrolink or expect to walk half a mile from the garage, must allow plenty of time to park.
Barnes definitely has a get-there-early culture for clock in. Mercy has a just-in-time culture, they would prefer that you appear at the time clock at 0659.
Mercy and SSM are both considered nice places to work, all the supplies you need and easy to make things happen. SSM has more aggressive pay incentives. None of SSM's hospitals are big, I'd say the biggest has less than 500 beds. Mercy has two very big hospitals, the main one on Ballas and Mercy South, formerly St Anthony's. The main hospital is very easy to get to from almost anywhere in the metro area and it's easy to park and walk in.