The ENR rankings are based on revenue. Therefore thier mind set is "the bigger the better".
Lower on the ENR list just means that the company's billable revenue is less than the upper firms and has nothing to do with employee satisfaction.
CENews rates companies based on responses to surveys about the the company, its atmosphere, benefits, worker satisfaction, etc. for its "Top Firms to Work For" and limits the surveys to Civil firms. Thier sister magazine Structural Engineering does the same thing for structural firms.
As jeb said these smaller polls are limited to the actual companies that voluntarily participate in the surveys whereas the ENR listing is based on publically reported information. Both are good starting places to get a list of potential employers.
Just pointing out the differences.