At this point it looks like ITER is hampered by it's relatively old supeconductor technology (ultra low temp/moderate field strength traditional magnets vs high temp/high field REBCO magnets).
ITER has been designed with relatively conservative magnet technology and will very likely provide the physics results that need to be understood in order for fusion power to become a reality. This includes experimental tests of the physics of plasmas where the heating is dominated by high energy alpha particles rather than external heating. This is a regime that's not yet been studied in a laboratory and there is important research to be done there.
Commonwealth is pushing the envelope of high temperature superconductor magnet technology and is relatively high risk compared to ITER's magnets (and this is a good thing). Lots of ITER technology will be useful to Commonwealth even before ITER turns on. For example decisions about which low activation steels and the huge amount of physics work that's already gone into planning for ITER.
I think the most likely outcome is that both accomplish their goals and contribute to making commercially viable fusion energy a reality in the future.
The tritium extraction and processing is a whole separate multi-story building full of first-of-a-kind equipment which will be 1:1 transferable to any breeding fusion reactor.
The work that ITER and IFMIF will be doing on material lifetime and handling under heavy neutron bombardment - a really substantial engineering problem - will be fully transferable.
The work on first-wall material which has to handle the neutron flux, very high thermal loads, and not poison the plasma when traces of it come off is also fully transferable.
Basically everything that's really new about ITER except for the size will work the same way on an HTS based machine.
I'd say about 2/3 for the science done at ITER would need to be done for any D-T fusion device, another 1/6 applies to all similarly configured tokamaks (i.e. it's less less relevant for stellarators or spherical tokamaks), and 1/6 is ITER specific (high estimate TBH).
If things run according to schedule (obviously questionable) then in 2025/2026 ITER will have first plasma and CFS will be on schedule to start building SPARC. CFS is being very clever in doing all their magnet design work first - investors are funding it because even if they don't get either SPARC or ARC funded and built, they will at least have some very useful IP on large HTS magnets which is bound to be worth something.