An aluminum fork will be lighter. That's the only difference.

If they're both designed well, a steel and an aluminum fork will function equally well, as stated above by others. My steel Specialized frame from the early 90s has an aluminum fork. Very light, very comfy. I replaced it with a Profile Designs carbon fork with a threaded steerer for sh*ts and giggles. Besides saving 20g (negligible) there was no difference in ride quality. I've since switched it back to the aluminum fork.

Is there a reason not to go with a carbon fork? If you move to an unthreaded fork steerer, you can save a good deal more weight than I did.