I played guitar for several years in the eighties and nineties and mainly played rhythm guitar, not so much lead. I took several years off of playing too. Now I'm back into playing and am practicing at least an hour and a half a day sometimes more. My rhythm playing has gotten back to a pretty good pace already, but I am also working on lead now.

I do use a metronome, and I do tons of excercies from Troy Stetina, John Petrucci, Paul Gilbert, Guitar World, etc.......

The problem that I have is that I can play much more efficient and faster when my left hand is fingering in the ascending direction. (example=1234, 123,13,12,14,134, etc...) , but when I go backwards or descending I play a little sloppier and not as fast. (ex=4321,431,etc....)

I do play slow and then speed up.

On chromatics I play at 120bpm on 16th notes clean back and forth. Forward on it's own I can play 150 bpm on 16th notes.

Should I just practice mainly on descending since it is much weaker.
Thanks in advance.
If you have an obvious weakness, target it and correct it. What's funny is that you may practice this so much that in a little while, it is actually your strongest technique. To practice, start slow (always) and concentrate of being clean. Only when you can play PERFECTLY at a speed should you try it faster.
bgc is right, i'm currently working on my ascending playing cuz its not as good as my descending playing (so between me and the TC we have a decent guitarist) but yeah, he says start slow, i would say just start at a speed you consider comfortable that you can play cleanly at.