What’s the diff between molded and screw-in cleats? Does one give better traction than the other?
The newer molded ones seem to be lighter than the screw in.
I’ve always been one to go with molded. I heard from numerous coaches that skill players should always go with molded. Not sure for exact reasoning, but that’s all I can tell.
Personally, from my own playing experience:
Molded cleats offer better traction on well kept grass and FieldTurf fields. Probaby due to the higher number of cleats on each shoe. They also tend to be a lot lighter. My personal shoe of choice.
Screw-in cleats offer better traction on long-grass fields, fields with loose soil, and wet fields. The longer/larger cleats with increased space between them allow the cleats do dig in deeper, at the expense of greater weight. However, they also offer the ability to change the length of cleat (up to the maximum allowed by the rules of the given sporting association). Screw-ins will also work well on FieldTurf as long as a short cleat is used.
Blinky, from what I remember you’re out on the WestCoast also. If you are looking to pick up a set of cleats and had to pick between the two, I would suggest going with the screw-ins due to their versatility - especially in the weather conditions out here. What are we at now? 50 something straight days of rain? I’ve lost count. 
I have heard that some foot injuries (lis franc sprain in particular) are more common in the “speed type” molded cleats that many wr’s and db’s wear due to their lack of support and rigid sole. Has anyone else heard this?
I was thinking of getting screw ins but would I have to change the length when I go from a turf field to grass? And also, what do the pros use?
If you are using the shorter studs that come with the cleats and the grass is dry - no. The only time you’d put longer cleats is if the weather was bad and you needed more traction. Only takes a couple of minutes, though.
In my VERY short CFL stint, the majority of the other DBs preferred the speed TDs (molded) but had both molded and studed (along with turf - for that ‘great’ astroturf field in Sask).
My preference- Molded all the way. More cleats on the actual shoe, easier to deal with (don;t have to interchange cleats) and are lighter.