What events will you be aiming for?

Choosing an event or particular field of racing has a big impact on which craft to purchase. If you are aiming for solo multisport races then a fast light single kayak or ski is the way to go. If it's the AROC or Max Adventure type events where craft is supplied (generally heavy plastic sit-on-tops) you may be better off buying a sea kayak or plastic sit-on-top.

Where do you live and where will you be training or paddling most of the time?

If you will be training in exposed area away from the shoreline then you need to take this into account. You never know when you may take that unexpected dip! Ocean skis are fast and can be unstable but you can always just climb back on (practice first in clam water first). On the other hand sea kayaks are stable, but not as fast. You have a more relaxed paddle and with a few lessons on how to Eskimo roll can be a great skill to have. And reassuring when far from shore. If you are going to be training on clam waters or rivers then the multisport kayaks are a good option as they are fast and light.