You're pretty on-topic. All of those are entry-level STAX models.
The main reason to go electret first is the crazy low price. I got an SR-30, SRD-4, and Lepai LP-2020A+ for under $100. When I got the SR-207 and SRM-212, it was a little over $300. (All used.) But I was lucky. Since the SR-207 was discontinued, the SRS-2170 price skyrocketed on Amazon and eBay. It's pretty much $1,500 and up now! (Though it's lower on amazon.co.jp.) The used market is a must, and you have to be patient to find better deals. (Unless you just decide to go with its replacement, the SR-L300.)
As for sound quality, I think all of us would agree that proper electrostats are a major upgrade from electrets.
However, I still miss that SR-30. It had a harder, tighter sound that worked well with exciting music. Electrostats (I'm sure you know that electrets work like electrostats except the charge is permanently applied instead of being supplied by the amp) have a stereotype of being "ethereal"...but it actually sounded more "solid" than most dynamic headphones I've heard! I also didn't think the SR-207 sounded ethereal (in the wispy, light sense) either. I'm still trying to make sense of that term.
I wonder how many others experienced extreme bass distortion at higher volumes with electrets. I always had to reduce the bass and/or volume when that happened. This was true whether it was being driven by an electrostatic amp or an electret converter and speaker amp.
Yikes!! I'm darn lucky I jumped on my 2170 system when I did, at $550-ish US.
I gotta hand it to Zeos, his review got me into them at exactly the right time. I love my baby Stax :)