1.77ct E/VS1 $39100.
1.96ct F/VS1 $38600.
1.76ct E/VS2 $35900.
1.65ct E/VS1 $34000.
Woohoo - I'm so glad you're going with Tiffany - the ring is one thing I have never regretted spending (my DH's!) money on!
I think Swanky brings up a good point... the big price point jumps for diamonds are typically at every 0.5 of a carat - so a 1.96 will be noticeably cheaper than say a 2.04, and the 1.92 and 1.96 will be quite comparable.
I personally would go for the larger F stone, given that the difference is not huge. The color difference between the E and F is really hard to tell... heck, when we were looking, I couldn't tell a D from an F!
Keep us posted on what you go with - congrats again... she's a lucky lady!