get the ones that are snap on/off, that way you can put them on around trees and buildings and take them off in fields or high wind conditions. gives you good options and still allows for most case uses.
The snap on ones also save you from having to remove the motor screws and worry about putting the longer ones back in mistake. No problem fitting in my case after I remove them.
That is the set I have and they work very well. Easy to take on and off and I like the two colors for orientation. I was able to descend vertically and hover less than a foot off the ground with no prop wash effects. I used the string but added a snap to make removing them easy.
I took a fishing snap swivel and cut off the swivel. The string was tied to the hole in one guard then threaded through the other guards. I attached the snap to the first hole and took the slack out with the string off the bottom guard hooks and tied to the snap. When I place the string on the hooks it is nice and tight. After I put the guards on I just connect the snap to the hole in guard where the string started.
I was going to make my own hook out of wire then remembered I already had some fishing snap swivels. You couldn't make anything better. It is small, light and even has a keeper.
Hmm, quad fishing now that would be an interesting new sport.
the string is to add rigidity to the props but I've never used it. as for speed and agility - sure the props decreases performance but only slightly. I've had the prop guards on since day one and I'm still on the same set of propellers. better to be safe then sorry