Either would be a good machine..... I personally would opt for the 2005 for body sytling in itself (and 800 power... but the 800 is a thirsty beast!), but I feel the 2005 is mechanically the better option also. As in any Polaris the front differencial is the source of most common issue (but it's also reviered as the best in the industry also!!). In the 2005 the front diff (central hilliard) uses parts that can be sourced through John Deer which are cheaper.....
if issue should ever arise.
As stated, just make sure everything works propperly! There have been a few threads in the past in regards to what you should look for and over.... try a search of the forum.
EDIT: Here is a recent "what to look for" type thread:
http://www.polarisatvforums.com/forums/atv-purchasing-price/50633-buying-used-atv.html
Just for comparison.... This last spring I bought the wife's 2005 Sportsman 400 for $3200 with 1700'ish miles, Warn 2500# winch and Eagle Plow system. It was (and still is) in very good shape for a 2005 (8-9 year old machine) Asking price was $3700. Ended up selling the plow system for $300.... so I basically picked it up for $2900 plus my travel gas (2 trips @ ~ $70). At the time $3800 was book value.
So don't be afraid to offer $3000 cash if it turns out to be a good and clean machine.... and actually get it for $3200-3500. This would be a good fair price for a clean machine.
Good Luck!