I've just started mountain biking and am having this dilemma too.
I've used road spds for over 3 years now, but am a bit reluctant to go spds on the mountain bike till I get used to the technical sections!!
So, till I've been on my course in a few weeks or until I get a bit more confident, I bought a cheap pair of studded platform pedals from wiggle, the
DMR V8 pedals and they seem good so far. I'm also thinking on getting some of the five ten shoes for a bit more grip in wet conditions.
I will ask the instructors when I go on my core mtb skills course for recommendations of spds easy to clip out of (for some reason I have a problem clipping out of shimanos - I must be a too feeble girlie!!) as I do intend to go clipless at some point.