Hi, I have worked both ER and postpartum, so hopefully I can be of some help!
As an ER nurse, I had 4-5 patients at a time with quick turnover. Sometimes they'd be low acuity, but other times I would get four new chest pain patients within a 20 minute span. In my opinion, ER nurses require strong assessment, documentation, and clinical skills, exceptional time management, and strong teaching skills because of frequent patient discharges. Each of these skills is beneficial in PP.
As a postpartum nurse, I had 6 patients. Within 2 hours of my shift beginning, I'd received report, reviewed charts, completed all of my assessments, administered morning meds, removed IVs, removed C/S dressings and got post op patients out of bed, and addressed requests like prn pain meds. The rest of the day moved by fairly slowly and involved pain medications, breast feeding assistance, teaching, a couple of discharges, and a couple of 15 minute admissions. The patients tend to be low acuity and fairly self sufficient.
If you can work as an ER nurse, you can absolutely be successful in PP. PP is nice if you're looking for a slower pace, routine shifts, and the chance to bond and form relationships with your patients and their families. If you are interested in OB but want a faster pace, higher acuity, and a chance to use more clinical skills, L&D may be a better choice. Both areas have their pros and cons, just depends on what you're looking for at this point in your career.
Good luck with whatever you choose!