Oh well, there isn't much to say:
Both options (North and South) had the same Estimated Time of Arrival (ETA) at the start. Weather-Routing sometimes suggested North, sometimes South a few days before the start. It was constantly changing from one option to the other.
South looked more stable. North had more chances of separating from the fleet, i.e. better tactical position.
How to tell which one was right? - I took a good old navigator's tool and made my choice: I rolled those dice I have in my Sextant case (these are used for calibrating the mirrors, but you can also solve unknown situations with them).
My tactical strategy was correct, there where only a few ships following me in the North. Weather developed differently, so I lost too much way against the South during days 2 to 5 of the race. I was 650 nm behind. I could gain 550 nm back, so this was 100 nm too much.
As Klaus had some real sailing to do in Greece, I also have his Earlhill to my disposal, so I sent this ship on the other option. At least Earlhill gives me some confidence in my navigation skills.