I think Crockeo is right, the updates are bounded as described, but as for your question...
Quote:
My first goal when making a game is to reach a constant logic rate...
This is probably the mindset that makes these two methods seem mutually dependent. You say you want a constant logic rate, and this can certainly be done in Slick. However, most of slick is designed around the idea that a game may have a variable logic rate. This is done through the variable "delta", the last parameter of the update method. It is a long integer representing the number of milliseconds passed, and can be used to time things like movement. This allows the game to run on fast machines and slow machines without a speed change in the games logic, while still allowing the game to take advantage of the speeds of a faster computer.
If you find it easier to use a fixed logic update interval, and are not worried that your game will be too intense for slower systems, then you can continue to use a fixed logic interval. But for many, these methods are quite useful separately.