Modular means that the cables can be removed, so you can use only the cables you need and not have to cram the rest into your case. It's mostly to make building easier, and possibly to end up with a cleaner-looking build, but in some situations it can improve airflow to have fewer cables in the case. It's up to you if you want that feature. The CX500 and CX500M are probably identical otherwise.
Looks like the Seasonic S12II is actually a fantastic unit. I wasn't sure, because I hadn't seen coverage of it before. Seasonic is generally one of the best, but it's a bit uncertain to judge quality based on brand name alone these days. It's not modular though, which is kind of a bummer.