The 2001 X1 is a good choice and possibly better value as most of the mods / recals are done as standard so there's less to spend on them.
However I'm an out and out S1 addict (1998 S1WL, arguably THE tube framed Buell) they're hard to beat for an out right raw scratching / hooligan machine, very flickable and very easy to shift your weight around on due to the narrow seat . They are more expensive especially the white lightning (pearlesent white) However the 1998 S1w or 'lightning strike' is less rare and cheaper with similar spec, different colours. The down side is that s1's need a bit spending on them to fix some of there niggles, once done though they are well sorted and pretty dam reliable.
The Orange one in 'for sale' has had many of the mods done by Maz and has been loved by a member of this club.
There has also been a silver one on E bay recently for 6k, again part sorted.
S1's can eat money but a sorted one is a sound investment that will appreciate in value ,not that that makes one fart of use coz you'll probably never part with it
Good luck with your search