I would say it depends on your travelling pattern (intra-European mostly, transatlantic, etc) and what you want from the FF programme (status, upgrades, free flights, etc).

Some points:

If you have a UK address you will not get AA miles for discount coach on BA nor on EI. No miles for BA transatlantic in any fare class.

Flights on LH except transatlantic in coach accrue only 50% mileage with UA.

To maintain status on AA you need at least four flights per year on AA metal.

The 100% bonus miles you get as AA Platinum applies to all oneworld flights. UA's 100% bonus for PremExec is only good on UA code, UA metal (or UA code, LH metal transatlantic).

All (eligible) oneworld flights count toward electronic upgrades with AA which can be traded for miles.

These are just some examples of things to consider. You will probably get better advice if you tell us a little more about how you travel and what you want from the FF programme.