The fat PS2 is probably better for soft-modding anyway. You can buy a network adaptor, stick an IDE harddisk in it, and run games off it with super-quick loading times / be spared the hassle of messing around with dozens of burned discs.

You can do the same with the USB ports on the slim, but the PS2's USB ports are only USB 1.1, so they aren't nearly as fast as IDE.