If you're only spending £5k per annum I am not sure what you aim to get out of even the free BA Amex as it will take you ages to earn a decent number of Avios. And the paid BA Amex now requires a salary of £35k to be eligible but it would be expensive if you have no aim of spending £15k to earn the companion voucher.

That said the free BA card does help to prevent your existing Avios from expiring. But then again a transfer of MR from the basic free Amex Rewards card would do the same.