I'm with 1989Pre. Trail runners for sure! If you're using toe clips and straps make sure they don't have really gnarly lugs on the soles, though. That can make getting in and out of the straps more difficult.

I REALLY like my Asics Gel Enduro 5 that I got on clearance.

The current Gel Enduro 7 looks like it might be a little luggy on the Asics site but they don't show the sole so I might be guessing wrong there.

I might add that I've had problems with some trail runners on some aluminum frames. A pair Saucony shoes and a really old Asics pair I used to wear had such wide heels that they'd clip the fat aluminum chainstays on my Trek 4000. It was just a minor annoyance, really but might put you off if you're gonna race in 'em.

Not having to change shoes in a sprint Tri is quite the bonus in my book.