confab
I really don't think that drag (more of it) is going to be an issue.
Keeping the banner attached is going to increase the rate of descent for one thing, to the extent that you may not have time to complete any checklist, not have the chance to restart the engine (if possible), nor will you be familiar with the handling characteristics or pitch attitudes required.
For me the banner has to go soonest - btw you shouldn't be towing a banner or flying over an area if you cannot land clear, not under UK law in any case.
When you hit the sea you will stop bl**dy quickly - when the wheels and prop hit (esp if windmilling) the deceleration to rest will probably be about 1 second - how much more quickly do you want to stop?