The main factors, as I see them, are
- Uncertainty around whether the merged program will be 3-tier or 4-tier, and if 3-tier what happens to the US Platinums (i.e., do we merge up to EXP equivalent, or down). I'm considering buying up to CP from Platinum just to guarantee I don't get merged down.
- Do you have a use for the CP upgrade certs? The domestic-upgrade situation for Platinum vs. CP isn't hugely different except on the most elite-heavy routes, so the CP upgrade certs are the main differentiator.
- Uncertain whether the new program follow AA's or US' lead on domestic upgrades. AA's program only gives complimentary upgrades to EXP, and everyone else needs to buy/earn and use upgrade stickers. Suddenly losing complimentary upgrades would really suck, and buying up to CP seems the safest way to guarantee they continue.
(right now I'm leaning toward buy up, but haven't yet done so -- will likely do it around Christmas)