If it's at all possible, give the variable RF a try (I don't know about availability in your area, but just in case). I have both the 45g and the variable 87U, and while I use the uniform one during my off-time (it's much more snappy and fun, not to mention better for games), I use the variable on my work computer. For long lengths of typing, the lighter weighed pinky keys are just fantastic.
Also, double check your wrist angle (they should be straight), the keyboard angle, your wrist-rest (if you use one) and your posture, etc. Those can all contribute to finger fatigue, making life harder than it has to be.