It's after 1am, nearly bed time... mid-Autumn holiday on Monday, although I'll be working anyway
The internals & performance of most Apeks regs, with the exception of the Flight, are fairly similar. Tek3s are closest to FST but designed as a left and right pair for better hose routing on doubles - not so handly for singles
It depends what you think you're going to do in the future but I wouldn't be buying dedicated doubles regs at this stage of your diving if I were you. Most people wind up having a dedicated single tank reg still anyway even when they go to doubles
I would suggest looking at an FST, the performance is as good as FSR or Tek3, but they're better suited to single tank use, and you can use them as a pair for doubles later if you want
Alternatively you could get a DST or DS4, and convert it to a stage/deco reg later. I guess by 'non-standard LP port' you mean the 1/2" port, which was only on the earlier ATX series, not the current XTX. There's no benefit to the 1/2" ports and the downside is that you need to use either a dedicated hose, an adaptor, or plug them and use another port (the latter is easy to do on DSTs but not so on the models with fixed port positions)