The most straight forward rendering of Galatians 3:24 is "...justified on the basis of faith." The preposition "ek" means"out from or arising from" faith. And as the OP correctly notes, the question is whose faith, ours or Christ's faithfulness. The effort to present "Christ's faithfulness" in both of the OP options is bogus.
Are we justified because of our faith? No, of course not. We undergo the washing of regeneration to be "made righteous." Thus we are justified by receiving the reconciliation provided by Christ's faithfulness in the sacrifice of His life on the cross.
The question I have asked again and again is why does God credit as righteousness the faith of believers per
Romans 4:23-25? I do not think any Calvinist has provided a sensible answer. Or why does
2 Thessalonians 2:13 say God chooses individuals for salvation through faith in the truth?
In summary we were justified on the basis of Christ's faithfulness.