An Amex point will hold its value better. If you transfer them to Avios and Avios devalues, you have lost out – you can't convert them back. Keeping them as Amex Membership Rewards points means that you have alternative options.

Transferring American Express Membership Rewards points to British Airways Avios can be exceptionally worthwhile, particularly for those who understand how to maximize the "sweet spots" of the Avios reward chart. The standard transfer ratio is typically 1:1, and Amex frequently runs promotional bonuses that can increase this to 1:1.3 or higher. Avios are most valuable when used for short-haul "Reward Flight Saver" trips within Europe or for domestic flights in the U.S. via partner American Airlines, where the points-to-value ratio often exceeds 2 cents per point. They are also an excellent choice for upgrading a paid Premium Economy seat to Business Class on long-haul British Airways flights. However, Avios may not be the best choice for long-haul economy redemptions due to the high "fuel surcharges" and taxes applied at major hubs like London Heathrow. If your goal is a high-value Business Class seat on a partner like Qatar Airways (using "Avios" as their currency), the transfer is often considered one of the best uses of Amex points in the travel industry.