I wouldn't use an auto-inflating lifevest for dinghy sailing. If it's a breezy day, it could be awfully expensive to replace the cartridge every time you capsize... I'd use the manual version on normal days and then use a full-on vest on breezy days (when you may be more likely to be surprised by the boom hitting you in the head).
Yeah, I meant to mention that, and forgot. But I'd forgo the manual version entirely. If I was dinghy sailing, I'd just wear a full-on vest and be done with it.