Light pedal or free-spinning pedal?
 

On my current bike I have Wellgo MG-8 SPD pedals, which are pretty light at 240g /pair, but they are plain bushed and feel like they are riding on molasses.
For my other bike I am thinking of going with pedals with ball bearings. But I notice they tend to be heavier. The Ultegra 6600 SPD pedals for example weighs 340g. Which is more important? light weight or less friction?

You won't notice the difference in weight or bearing type. You will notice a difference in SPD and SPD-SL, aka "road" vs "mtb" style.

As I understand it, you don't want pedals to spin super-freely - you want them to be sufficiently free for hang down, so they're easier to get your cleat into, but not so free that they start spinning if you miss the first go. I degreased my very old Ultegra SPD-SLs last year, but I don't believe there were any ball bearings in them - I think they were all bushes

Speedplay Zeros have worked well for me for close to 20 yrs.

It's unlikely you will notice any performance Change from a pedal swap

As others have said. Weight and friction are pretty negligible. Float and surface contact are usually the big factors in selecting pedals for me. Making sure there is enough “play” from float to accommodate your knee/ankle anatomy while spinning will give you more benefit in the long run.

I am using Shimano sh-51 cleats, which supposedly gives you 4 degrees of float. If that enough or is it better to have more?

I used those or like that for a while, & AIR they were so light that they didn't hang consistently,

making clipping in more of a bother.

I think those have roller bearings, & cleaning & regreasing would likely make them spin nicely.

More float is only better if you need it. if you have no complaints about your current 4˚ cleats, more float is not likely to feel like an improvement.

https://www.competitivecyclist.com/s...pdp-tab-weight

208 gr. in stainless....

it’s really dependent on how much lateral movement your knee has through the stroke. Don’t get too technical about it and any float is better than zero. Then again some people are completely fine with zero and others develop knee problems. You should be good

I'll take free-spinning over lightweight every time. That's independent of retention systems, (which I mostly forgo).

+1 for free-spinning. Depending on just how like molasses your current pedals are, there's a good chance you actually will notice the difference.