Can't speak for you, but I would definitely *not* want to be in *any* kayak or ski for a first time on race day, provided that race means anything to you in terms of finishing or finishing well. You don't even know if your legs and bum will fit in the ski physically...
As for paddling a tippier than you are used to craft over 24 miles on flat water, you should know that it is not easy. If you feel even a bit of tippiness on the ski, you will lose energy to maintain balance. During my first nearly 20 miles race (the Blackburn Challenge) I used the Rapier 18 kayak, whcih is very comparable in stability to intermediate skis like the Epic V10 Sport. Towards the end of the race, after 15 miles hard work, I was getting tired and my balance was not good enough to maintain good speed. I would not have had these problems when not tired, nor would I have them in a more stable kayak.
That said, I was still faster in the tippier but "faster hull" kayak than in a slower and more stable one. But there is a cross-over point, which is different for eveyone, where getting a "faster hull" that is too tippy leads to results worse than you would get in a more stable but "slower" craft.
Your problem is that you have not had a chance to compare the ski to a kayak for your kind of race. So, tough choice -