If I understand you correctly, you're going to be doing single tank, open water recreational diving. Sometimes you'll be using regular air, and sometimes you'll be using a nitrox mix (probably 32%, right?).
Don't buy the Tungsten. It's not worth it.
The Nitrox XTX regulators include "oxygen-clean" first and second stages. The second stage face plate is colored a nifty green/yellow/black. Other various rubber and plastic parts are also colored green. The DIN handwheel on the FSR first stage is green/yellow. So basically there are several striking cosmetic differences.
If you're a recreational diver, your regs don't need to be "oxygen-clean." All Apeks regs on the market (and pretty much all other major brands as well) are ready to be used with nitrox mixes up to 40%.
Performance-wise, the nitrox Apeks regs are exactly the same as the regular Apeks regs.
For single tank recreational diving, the Apeks XTX200 is a good choice. The FSR first stage is nice. That being said, you probably couldn't tell the difference between the XTX200 package (XTX200 + FSR first stage) and the XTX50 package (XTX50 + DST first stage). The guts of the XTX200 second stage are the same as the guts of the XTX100 and XTX50 second stages. Why pay more for cosmetics? There are subtle differences among the first stages (FSR, FST, DST, DS4), but from my limited experience, the most noticeable difference is where and how many HP/LP ports there are. Hose-routing on the FSR, FST, and DST are all great for single tank diving. The DS4 has fixed ports that aren't quite as conveniently located as the FSR and FST, but the DS4 is a wonderful value if you are a little more budget-conscious.
Just make sure that your local reg repair shop can work on Apeks regs. The regs will only breathe as well as the reg tech tunes them.
Hope this helps...