double strength are generally not needed as they don't provide additional energy savings. However, there are some benefits. The are a bit stronger. both panes are affixed to a spacer to form a seal. This keeps the argon gas inside the two panes. Leaking of this gas is called a "seal failure" (foggy glass inside) This is generally covered by warranty for 10 or 20 years but the manufacturer will only replace the glass, not the sash. (some exceptions). A dealer might cover the cost of installaion for the first few years. After that, it's your responsibility.

With wood windows, replacing glass requires a glazier who will charge $50 to $100 per window. You might be able to do it yourself, but it is tricky.

Things that can cause a seal failure include flexing during installation which puts stress (twist) on the window glass. Enough stress and the glass may even crack. Also expansion/contraction of wood framing around the window due humidity or drying out or even settling of the house can put stress a window as well as opening / closing hundreds of time. Double strength being thicker and stronger will resist twisting / bending and help prevent seal failure.

If there are extreme cold hot swings in temperature, the glass and sash can expand and contract more and cause seal failure. Because of this, Canadian building code requires double strength. The US does not.

Double strength are also quieter at keeping noise out. Oh, and they are a bit heavier to physically pick up.

Thanks for the info. So does any one know if these is standard on Marvin? This is what my dealer is telling me, but when I called Marvin, they acted like I didn't know what I was talking about...

Yes Marvin makes it standard on all their windows. Marvin is a much better window.