I used to sell thousands of Certainteed windows but, I changed occupations.
This is what I would recomend you do,
I would probably buy the mechanical frame with welded sashes. Once the frame is installed, shimmed, screwed into place, it's not going to move!
The most important thing that you need to worry about is that the frames are shimmed properly so the sashes have a nice snug fit.
A welded frame will have to have the same installation process. So, there is no advantage as far as fit or installation for a welded frame.
The sashes do all the work so, you should have welded sashes. They are attached to the glass all the way around and are very strong.
Both windows should have the stainless steel coil balance system.
Now* Low e glass with argon gas.
Here's the deal!
Low e glass is good for saving on heat and a/c. It also helps to reduce fading of curtains rugs etc.
Argon gas will dissapate in about five years or so and I have yet to figure out where it goes in a sealed unit but, it does.
You need to look at the cost of the mechanical frame, welded sash with Low e, vs the all welded with low e and argon.
I think you will find yourself saving some money!
Ps. I had Certainteed in my old house and now have "Harvey Classics" in my new home.
I don't know about thier sliding doors because the licenced manufacturer in my area did not make them.
The windows, as I have stated, are fine.
But, this does bring up a subject that you may want to look into.
Find out where these windows are going to be made and how many licenced manufacturers there are in your area.
There are three in my area. They must buy all the parts from Certainteed and meet thier standards.
I would be surprized if they did not stand behind any problems that you "might" have.