Hi Tartlet,
Most of my bookings are in advance. I do ask for advanced notice on my site, but that's mainly because I don't live where I work, so a little notice is important. Saying that, there have been a few occassions where I've taken bookings on the same day, providing I've had adequate notice, i.e. a morning call for later in the day.
Funnily enough, in my experience, the no-shows I've had have been guys that have made a booking on the same day. I've had a couple of bookings in the last six months, where they've called in the morning for a booking for the early evening, and they've not bothered to turn up. On one occasion I was already at the flat anyway, so I didn't mind so much. But the second time it happened I'd made the trip into London especially, and I was none too pleased.
I prefer bookings taken in advance, because if the gent changes his mind and doesn't want to see me, then he either cancels with a bit of notice, or he doesn't confirm by my alloted time in the morning. Which is fair enough. I move on with my day.
And no, I don't find much difference with clients who phone, rather than email. If there is, it's only with the duration of the booking itself. Sometimes those who email for an advanced booking tend to book for longer.
That's just my experience.