2 of 6 added 452 characters in body

Ruby, 66 (shows only which one)

a=gets.gsub(/./,?1).to_i
b=gets.gsub(/./,?1).to_i
puts'sab'[a<=>b]

Prints s for same, a for string a, and b for string b.

Ruby, 184 173 (also shows by how much)

a=gets.gsub(/./,?1)
b=gets.gsub(/./,?1)
d=->e{(Math.log10(e.to_i)+1).floor.to_s}
if a==b
$><<?s
elsif (c=a.sub(b,''))!=a
$><<?a+d[c]
elsif (c=b.sub(a,''))!=b
$><<?b+d[c]
end

Sample run:

c:\a\ruby>stringcmp
testing123
testing
a3
c:\a\ruby>stringcmp
codegolf.SE
code golf . stack exchange
b15
c:\a\ruby>stringcmp
test
test
s
c:\a\ruby>