Mountain Biking - what is right for me, clipless w/ or w/o the platform?

I have a pair of shimano m180? mtb shoes on order from the employee purchase program at work. and i'm just starting to convert to clipless from my flats. right now i've got some old crap shoes that a friend gave me with the shimano clippless pedals that came on the bike. I ride am but lean toward the fr side. I am willing to make the weight sacrifice for the platform feel, but i'm wondering if there are any other unforseen sacrifices i'll be making going form regular shimano clipless to the shimano DX (with the platform)
by the way i am soom going to be doing a 6 to 12 hour race in tahoe including a 23 mile ride, if that influences your input. thanks for your opinion in advance.

I've never been a fan of the combo deals, but they have their place.

My experience has been that the platform interferes with the binding mechanism, making them a not-so-great clip in pedal. But at the same time, I found them almost impossible to step on as a platform and NOT unintentionally clip in, making them a poor platform.

There are also the flip-flop combo units, such as the 324, but they are compromised as well. The binding mechanism on those stinks (similar to a 515 or 505 pedal I'd guess), and is very "sticky" to engage and have a soft, undefined click-out point. Additionally, the platform is not very grippy and the there is no default side UP -- that is, when you put your foot down on the pedal, you'll never know whether you're going to get the platform side or the binding side.

That said, I put up with a pair of 324s on my commuter bike because I like the utility, and use non-platform clipless for everything else. In the tool box I have a pair of agressive platforms I can install at a moment's notice.

I have a pair of shimano m180? mtb shoes on order from the employee purchase program at work. and i'm just starting to convert to clipless from my flats. right now i've got some old crap shoes that a friend gave me with the shimano clippless pedals that came on the bike. I ride am but lean toward the fr side. I am willing to make the weight sacrifice for the platform feel, but i'm wondering if there are any other unforseen sacrifices i'll be making going form regular shimano clipless to the shimano DX (with the platform)
by the way i am soom going to be doing a 6 to 12 hour race in tahoe including a 23 mile ride, if that influences your input. thanks for your opinion in advance.

I have the same shoes (M180) - they're good and sturdy. :)
I use the M545 pedals for DH.
(photo: http://www.chainreactioncycles.com/Images/Models/Full/771.jpg)
If I'm on course and I need to clip out, naturally I'll need to get my foot back on the pedal as fast as possible. Sometimes I don't hit the clip-in the first time, but I do have enough platform there that I can have my foot on something.

The platform/clips work fine for me.

For XC, I'm on M540s - no platform needed. I guess the assumption is that I won't be (very suddenly) headed down a really steep rocky embankment while NOT clipped in and need something to put my foot on. This is what the platforms give me.

Hope this is helpful.
-sunny