I find the lock jack is either on or off for the most part. I found it easy to go real slow with but after that for me it was pretty much a free fall with an abrubt stop.
The spiderjack seems to have a really good mid range. Descents can be made at something faster than a snails pace. The biggest problem for me is the thumb control when trying to make a fast descent or jump it's just not intuitive for me where the lockjack just seemed more natural. Overall I think I can climb smoother on the spider jack but it takes me more thinking. As far a self tending I never used a real small line on the lock jack but even with 1/2" it worked well enough, the spiderjack I have always used 11.7 or 11mm and it tends great once you get the hang of how to use it.
I actually find myself using a hitchclimber in a similar way actually feeding the rope into the hitch and it helps it to tend at much lower heights at least for me anyway. I climbed back to back with my spiderjack against the hc and once I was up 25-30' I could get the hitch climber to tend pretty close to the spiderjack but that was with a real loose hitch that has to be set to be sure it will hold.
Other than the whole mechanical learning curve the only other things that took me some getting used to were the performance loss with no real fairlead system and having to learn to not ease yourself into the device when resting. Kind of back to that on or off type thing. Hope this helps, I am not a diehard mechanical device user with a lot of experience with them but I have logged a few hours with some of the mechanicals. I will say if I had to pick one of the 2 I would go with the spiderjack
