Hi Pebble, I am a devoted SPD user, well Crank Bros Candys anyway, but have recently put a pair of flat pedals on my hardtail to "improve" my skills. I still run Candies on my Full-Suss so I can give you a bit of a comparison as I see it.
Clipless pros: More power, more security, feet won't come flying off the pedals - not always a good thing in a fall but essential on a descent or technical climb. Can sit and spin, especially on a full suss. More efficient and less tiring over a few hours.
Flats pros: requires (surprisingly I thought) a smooth pedalling style, easy to get off when falling, (unfortunately easy to fly off when descending or climbing). Can't just sit a spin all the time, especially on a hardtail, have to attack things and gives you a much harder workout. Teaches you good form when doing rear wheel lifts/bunny hops etc. Actually quite good fun and encourages a "play" mentality in me.
Clipless cons: Attached to bike, allows you to be lazy (I never realised how much clipless lets me get away with by keeping my feet attached to the bike).
Flats cons: Eat shins for breakfast as the several puncture wounds on mine will attest! Less secure feeling especially if used to clipless. Much more dificult to do technical climbs on, harder work generally. I find hill starts really difficult on them, especially if steep. Slower cadence for me, a few of the guys I ride with found the opposite but I tend to spin much more than them generally.
Having ridden clipless for about 4 years off road I will admit that I threw all my toys out of the pram several times when I first started using flats (about a month ago), its was like going back to being a complete novice again and I can't remember the last time I rode that slowly or climbed so badly. I put them on as I did a skills training course doing things like bunnyhops, rear wheel lifts and the instructors said we should try it with flats as it would show us correct technique, so on they went. I am pleased to report that I have gotten more used to them and actually enjoy riding them (on rides of two hours or less) and I can bunny hop and lift the rear wheel. I will be keeping them on for a while yet as they are definitely improving a lot of things about my riding.
But..... clipless still rules as far as I'm concerned, I was on my full-suss for the first time in a month at the weekend and it was nice to spin at my usual cadence and to absolutely hoon down stuff feeling part of the bike.
If the technicality of the trail spooks you then there will be a tendency to worry about being attached to the pedals, when I first started I was exactly the same. Once my confidence and skill grew it improved. These days if I don't like the look of something it doesn't really matter what pedals I'm on I most likely won't ride it.
I still have moments when I can't get out of the pedals, it happens to us all. Sometimes things happen so quickly it wouldn't matter what type of pedals you have on you will hit the deck.
I would personally say that you stick with the clipless especially if you have been riding a while as you will have conditioned yourself to them. If you decide to change to flats I'd pick up a cheapie pair first and whatever you do, under absolutely no circumstances try to ride on them in standard cycling shoes especially if they have cleats on, you need a set of flat soft soled type shoes (I use an old pair of Vans I had) otherwise you won't stay on them. I would also be prepared for a bit of a tradeoff between ability to get off the pedals quickly and the ability to do anything else on the bike, at least until you get used to them! You'll also need to ride them for a while to give them a fair chance, the only snagette is that if you don't ride clipless for a while you could "forget" the unclipping movement. I have tried to haul my feet straight off my Candies a couple of times since getting flats luckily no harm done!