Differences Between a Cast Iron or Steel Base for a Belt Sander?
I can't imagine you would be applying enough pressure on a belt sander to worry about cracking.  

It is definitely true that a heavier sander will not vibrate at the high frequencies generated by normal sanding.  

Steel is strong and light, so it might be better for a portable belt sander.  But cast iron might be better for you in a workshop.

In the end, with light sanding, you might want to do some shopping and look at the features then find the best price to performance ratio to make your decision.