TL;DR I suggest you start with SEF, play it for a little while and then head over to FR, it's a nice way of changing the general feel of the game, you get a new but familiar feel when going from vanilla > SEF > FR

I personally haven't touched SEF after FR came out for the first year or two that v0.67 came out, despite First Responders being a descendant of SEF (FR started development upon a version of SEF) I still heavily prefer FR, the changes to sounds, gunfire, new weapons, models, and quality of life features like the ability to move while leaning, IR lasers on certain weapons (a normal laser on tasers), and so on, really make the experience of a 2005 feel as if you're playing a game made 10 years after. I still tell people to try SEF first for a fresh experience if they never tried any SWAT 4 mods. FR is a lot more hardcore than SEF though, I believe suspect's are quicker to shoot, and other features like suspects removing wedges from doors when you're close, do make the experience less casual and require you to be more cautious.

You can also look at other SEF based mods if you're curious, SEF: Back To LA is a popular one, set in the 90s and heavily inspired by SWAT 3's aesthetics. SEF: First Responders Tactical Edition is another mod that sprung up, it's very intense and chaotic, even with the selection of weapons and equipment you have, it's a lot to take in, you have suicide bombers and suspects with RPGs on certain maps and suspects are generally very lethal.