Replacement Vinyl Windows in Chicago

I have a two-story colonial in a NW suburb of Chicago, which has 16 windows and one sliding glass door. The only two quotes that we think are legitimate so far are for $5700 and $6900. The $6900 guy is using "Thermoplast" windows, but I believe the brochure he left was from Republic. Anyway, do these seem like a reasonable price...$430/window (I'm including the sliding door in this calculation)? That seems pricey to me, but we like that guy and his windows and his warrantee are the best so far. His company name is Creative Exteriors. The other company we'll be talking to is Champion, but I expect them to be like Sears- way overpriced, with an hour-long presentation. We're also considering buying from Menard's or Home Depot and having a trusted contractor install them.
Taking a step back, here's what I would expect: $170/window cost plus, say, $100 installation = $270/window * 16 = $4320 + $1200 for the sliding door = $5520... thoughts on this are greatly appreciated.
On the window design, we want the flippable, cleanable, but importantly, well thermally insulated. My largest gas bill (~2000 sq. ft) last February was $230... I hope to cut this down to $150 or less. Again, any thoughts on this? I don't want windows that have a metal strip in between the panes, and I want argon filled. Also, fusion welded at the corners... pretty standard stuff I'd imagine.
Thoughts on any or all of the above issues are greatly appreciated.
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