Which ext for SADX & SA2 models?

[sonic65101]

Only Sonic Adventure 2's model files are stored in PRS files. The models in Sonic Adventure DX are actually stored as C-Structs in the game's executable and DLL files. I recommend you use the SAMDL program, part of Sonic Retro's SA Tools kit, to extract them. Here's a list of the model locations and addresses. Keep in mind that the list is incomplete, though.
https://info.sonicretro.org/SCHG:Sonic_A..._Locations