S4 vs. S6 choices

[Brian Pleet <pleet@sfu.ca>]

I'm upgrading from a 1990 90Q20V to either an S4 or S6. I have received
offers on several vehicles. I'm not sure of the differences (performance
and/or appearance) between the S4 and S6? I've received the following
offers, any commentary on them would be appreciated.

1997 S6 0k 36 month Audi card @ $61,000CDN
1997 S6 2.5K 33 month Audi card @ $58,000CDN
1996 S6 15k 13 month Audi card @ $52,700CDN
1995 S6 23.7k 36 month Audi card @ 51,100CDN
1994 S4 44k 15 month Audi card @ $43,500CDN

P.S.
a)Can you justify $18,000 difference between the oldest and newest car?
b)Is the extented warrantee offered by most Audi Dealers valuable as a
trade off against a shorter Audi card?
c)Is there a performance difference between an S6 with 15' vs. 16'
wheels?