Robert is more popular. http://www.behindthename.com/top/search.php?terms=robert
http://www.behindthename.com/top/search.php?terms=roger
They're both Germanic names http://www.behindthename.com/name/robert http://www.behindthename.com/name/roger
Robert sounds much more classier and is more popular but I wouldn't use those names on my kids.