Which one do you prefer, for what reason?
Does lube really do that much for the old slides? I was surprised at the amount of friction the old slides generated.
They both have their advantages and disadvantages. Depends upon the use case.
Old slide advantages:
- able to distribute a load over a longer travel distance
old slide disadvantages
- outside track easily bent and made useless
- heavy
- can take up more room than you think
- again, really heavy
New slide advantages
- lighter
- less likely to bend and be immediately useless
- longer lengths!
- two very different configurations possible
New slide disadvantages
- hard to get long stable throws. You can’t depend upon the track feeding back into the plastic slider once it has left the housing
- point of hold over a small distance of the plastic slider is not as good as the metal in some configurations (e.g. on an angle)
- plastic slides can get lost (comes with two kinds, but you only use on type at a time so the other tends to get lost)
- plastic slide holes weaker than metal contacts and can get worn out
- plastic slides not sold separately
Disadvantages of both kinds:
- the torque difference between 12 tooth and 36 tooth gears is very great making linear slides slower than you would like since you go down to the 12 tooth gear often. More gear options are available in other lift types. Need a happy medium with a high strength option placed on the “in between” row of holes.
Agree with that. I tend to use old ones when you need to use rack gears to power something, and new ones elsewhere.
New “turbo” gears will help with this 
why do you use the old slides when powering rack and pinion gears?
Mainly because we have only a few new ones available and the best place for the old ones is under direct power because they seem to have more friction with any kind of offset loading.