What rifle has more of a chance to replace the m4 the Remington ACR or the FN USA SCAR?

None. The M4 works pretty well for our current purposes, and funding is going to be drying pretty quickly once we draw down in Afghanistan. Replacing a perfectly serviceable weapon will be a luxury that the USA/USMC can't afford (Special Forces/Operations types may be able to acquire them, but it won't become a regular service rifle).