How can one choose between "tunable" and "tuneable"?

I think you will find the last part of this page helpful. 1

tuneable: It is still not too old-fashioned to cling to tuneable, though sentiment is moving towards the Americanised variant. But by choosing tunable you need to be consistent, which ultimately means adopting aging instead of ageing. Scaleable does not become as clearly readable as scalable -- which is why the British prefers to keep the -e- in its rightful place. Americans only have the choice of tunable, luckily British English retains the -e-. The US variant looks much too "fishy" if you ask me.