The A4000 is definitely going to be the budget buster no matter how you look at it. But as tokyoracer said, it does have the AGA chipset built-in as standard and is the most powerful of the Amiga lineage.
One factor you also have to consider is space. If it's not at a premium then the A4000 is the one to go for.
*Addendum to my previous post*
Yes, WHDLoad should also be considered so a bigger hard drive and the possibility of an external CD-RM (to store the games on) should be factored into the price.
