From a strictly miles maximising perspective I think that would depend entirely on the way you earned the miles and the upgrade cost, vis-a-vis the cheapest F availability you can find for your travel dates minus your J class cost

On the other hand, EK miles expire strictly after 3 years and can't really be transferred out so that might tip your decision...

For me it would depend on the cash charges as well - I find the best value is upgrading a J saver on a space available basis when F is nearly full as EK don't charge surcharges on upgrades but have extortionate redemption surcharges.