Might be pretty tough as they(as well as everyone else) has changed especially casement hardware at least several times. The sash locks have become sequential and the operators changed at least twice. Where are you located i used to sell Kolbe maybe I can help.

Have not used Kolbe and only seen them as displays, but the quality looked impressive. Pella on the other hand is some of the worst junk out there. I speak from first hand experience as I have installed top of the line Pella Architectural series windows and french door in my new addition. These windows are garbage and I will never ever buy Pella again. My issue with them mostly is the lack of attention to detail. Issues like hardware being screwed on crooked, bed miter joints. The overall construction is however functional. After I have spent massive amounts of time correcting them, the windows have functioned fine for the last 5 years. Still I will never forgive them for the product that was delivered.

I am only familiar with the Kolbe. But if warranties are the same, and the ratings have been certified (ask to see NFRC Certificate), then it's a matter of price over performance. The slight difference can make a noticable difference. Interestingly, the unit with the lower U value (which is better) has a higher Solar Heat Gain Coefficient, which is not better. The lower the SHGC, the less heat gain, and the less cooling costs will be. The difference would be hard to calculate. It would depnd on your HVAC system, and a few other things. The other issue is you are not comparing apples to apples, in regards to installed vs uninstalled. Can you get the Kolbe's mulled together and installed for less than $4K? 42 windows is a decent amount of windows, so I'd get a quote on installation. Wouldn't make sense to save $4k on windows, only to end up paying $6k for the install.

@Robin: Our Kolbe and Kolbe windows look great--in fact they look new, if they were clean, after 13 years. No problems with condensation between the panes. We had to replace one large window but that was due to installation error, not the window. The issue we have had is with service. We had some broken handles and some of the windows are difficult to close (casement, due to settling, I suppose, in earthquake country). Our GC went out of business so I don't know where they sourced the windows from. I called several local places that 'carry' Kolbe and Kolbe and they could not help me. I called Kolbe and Kolbe and they were impossible. When we had the window replaced, the new contractor was able to assist with the other windows (11 years later). Also, in retrospect, I wish I had gotten sliders instead of french doors. One of the handles has been dysfunctional (a K/K thing), and I think french doors in general are not very secure (not a K/K thing).