The GU Carmarthen contract specifies IETs from Hitachi, which I imagine is set in stone even with the recent delays in getting it up and running. Even more so when bi-mode is needed and IETs have a proven track record of delivering on this.
For Stirling, GU have binned the Mk4 proposal (don't let the new CGIs on their website allude you), and have been discussions between GU, CAF, and Hitachi about finalising a deal for an off the shelf new build if the ORR grant track access. So in other words, the ease of procurement with something tried and tested and lower cost will be key to what rolling stock is offered for any Stirling service (IETs or Civities most certainly).
I can see why the TPE Mk5as would be attractive in this instance if CAF can sort the issues out with them.
Even more so when negotiations with GU and the ORR, if my knowledge serves me correct, are largely only hinging on Network Rail's unwillingness to grant track access for more than 5 years (as opposed to GU's desire for at least 10 years). Having access to rolling stock that is already funded (with precious tax-payer cash), built, and gauge cleared across the whole network in question, would bolster their value-for-money case in this instance.
I wouldn't rule it out anyway if Network Rail are convinced to budge.