Marvin is the superior product, bar none. Performance,design, etc... It is just a better product.

Why?

The Pella is flimsy and not designed well..also leaks air. Marvin is designed MUCH better and is also much nicer looking.
Just because a window is fiberglass doesn't mean it's good. There are some high end vinyl windows that have better structural ratings than any fiberglass window. Those brands would be Okna , gorell, sunrise, Polaris Ultra series windows.
Pella vinyl is very low grade as is andersens.. They are made for those looking for the cheapest price same with Pella fiberglass. Marvin is much better.

Based on all the research I did, I would compare Integrity to Milgard Ultra and remove the Pellas from the equation.

I ended up ordering the Milgards only because they had interior color options from the factory (in my case, black), whereas the Marvins only offered a white interior. Otherwise I would have gone with the Marvins.

The lifetime warranty on the Milgard is a nice advantage.

Marvin has a lifetime warranty on the Infinity line now.

I think it translates to the Integrity line now.

I spec'd the Impervias and Integrity on a new community center project. The Impervias are what got installed. They are rather large sizes, combination of fixed and casements. The Marvins have a nice smooth fiberglass appearance while the Impervia has a slight texture to them.

Performance wise, the Impervias have better u values, more glass options (if you are wanting high SHG) and a pretty good infiltration number (better then the Integrity).
It also has full stainless steel cranks and hardware unlike the Integrity line. The Integrity looks a little nicer, but I suppose time will tell on how these Pellas hold up. I have the single hung Integrity going in on an assisted living facility that is starting construction now. Will be nice to compare the 2.

CWirick- How did your Milgards compare price wise to the Integrity windows?

For me on my house? Probably none of the above. Going with a Canadian fiberglass like Inline or Fibertec. Better product and ratings than any mentioned and not that much more in cost.

From the outside (i.e. finished surface) the Impervia is not bad looking.

When you see the bones of the product next to the Marvin, not as favorable.

Better yet, as you mentioned, is the Inline window.

@ Izerarc: My Milgards were equivalent in price to the Marvins, until I added the black interior option, which was a surprisingly expensive upcharge. But I didn't want white frames flapping in the breeze when the casements were open, nor did I want to deal with the unknown results of having a painter paint the frames on site. So I bit the bullet.