Clipless all the way! I tend to like a bigger platform but still clipless

On my road bike - clipped in.
On my Tri-Bike - clipped in
On my single-speed toe-straps
On my shopping/around town bike - platforms.

Both of my mountain bikes ?

O'Neal Azonic 420 platforms pedals. These things are light and bullet-proof and offer a ton of grip.

I still have scars from clipped-in mountain bike crashes. Sure it might get me to the top of the hill a few second faster, but even after years and years of cycling clipped-in, I am still not quick enough to twist un-clipped and dab a foot down in rock gardens.

Frankly the idea of a fast (30mph+) decent on a rutted trail while clipped-in is terrifying too.

I tried many different combinations of pedals and shoes ranging from SPD to egg-beaters and hated them all equally off-road. Actually, no, the egg-beaters were the worst, horrible things...

I ride with a group that is about 50/50 platform/clipped nowadays, there was a time when it was 100% clipped-in, but the crashes where the rider has stayed attached to their bike making the injuries worse have tended to persuade us that we should leave clipped-in MTB'ing to the racers and the casual riders like us are way better on platforms....

Personally I cannot recommend the Azonics enough, they are super grippy, very light and insanely strong. I have had multiple very high impact crashes and despite a crank-bending impact, the pedals are still great, if not a tad scruffy....

I should probably amend my previous statement and say that I've never tried *good* platform pedals.

However, I've never had a second on the road or trail where I wished I wasn't clipped in.

I run Looks on my road bike, tri bike, and my old roadie that's been retired to my trainer (recently changed from shimano road SPDs), and mountain SPDs in my trail bike, xc race bike, and my single speed. Being physically attached to the pedals is faster, more efficient, and makes me more stable on the bike. After about 3 rides I was used to them and haven't had problems unclipping when I needed to since then.

Flats all the way with shin guards for when you have to jump off your bike mid air.

Flats with spikes like crankbrothers or straitlines and a good pair of Five Tens or Tevas. I love my clipless on my road bike, but it's flats all the way for the MTB. You do have to be careful of shinburger though!

But he's also put me on Candies (which are like egg beaters I guess). I've been working on them all summer and I'm much better than I was but I was having chain issues Wednesday so that combined with the pedals meant I fell about 3 times. It was suggested by a friend that I go to flats.

It should have been suggested to fix your chain issue and keep the clipless pedals.

As for me:
SPD-SL on my road bikes
SPD on my mountain, cross and gravel grinder.

My avid biker buddies kept talking me into the special shoes and the clips and the locks, I just like wearing my regular tennies on flat pedals.

I actually started out mountain biking with clipless pedals and did that for probably 5 years. But as I got into racing DH mountain bikes, I ran into problems with very techy stuff where I couldn't get clipped back in if I dabbed a foot as the terrain was often too rough and I switched to flat pedals for that. And then I tried riding XC on flat pedals just to get better at riding flats and after a while realized that there is virtually no difference in power or efficiency, the flat pedal shoes are way more comfortable if I need to walk anywhere, and I'm more confident if I come up on something unknown or extra techy if I'm not clipped in. So now, for the past 5 years or so, I'm 100% flat pedals except on my road bike.

Also, my knees are already horrible.

As long as you set them up with sufficient float then they shouldn't effect your knees.