Readyboost works best if you're using one or two flashdrives that corresponds to the amount of RAM that's installed on your system. as such, i would recommend the 4 GB stick if you have 2GB or more installed on your system. if its less than 2GB, the 1 GB stick would suit you better.
in terms of pure speed, unless one of the sticks is a USB 1.1 stick, then you probably won't see much of a difference between the two when it comes to read and write times.
hope that helps.
See the struggle of the faithless lot as they negate their time
How low to sink to the depths of their frame of mind