Insulation is good as long as they are built to last.
Construction, insulation, and installation, make for a good job.
What I have in my home is a mechanical frame with welded sashes and Low-E.
You can spend more money for welded frames but, once they are installed, shimmed, and insulated, the frames are not going to move. (save your money)
Welded sashes on the other hand, are a moving part of the window and get all the abuse. It makes sense to have the sashes as strong as possible and have the glass attached to the sash all the way around.
Double locks make sense when your windows get to be around 30 inches in width. It is a good idea to lock the windows everytime you close them to keep true and square. You avoid problems in the future.
Locking screens are kind of a gimmick because the screens will stay down as long as we have gravity. So, if you have to pay extra for them, don't bother.
Low-e can help you save energy and help keep the suns rays from fading your rugs and such.
Argon Gas will dissapate in about five years. I would not buy it if it's not included.
Preparing the opening to receive the new window is very important. Make sure after you remove the old sashes, that you caulk around the window stops that the window is going to rest up against, before you set the window in so the window will seat into it. Proper shiming will make sure that the window is true and square and that the sashes have a nice fit to the frame. Using insulation between the new window frame and the wooden window jamb is a must. (I prefer fiberglass because you have more controll over it than foam in a can.) After the window stops on the inside are installed, I use another bead of caulk to seal the whole window.
If you do all of this, you will have a very tight window and no drafts!
Sorry to run on so long but, I want to make sure you don't waste your money and you know what to look for while they are being installed.