3 of 6 deleted 46 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, 184 173 146 143 (also shows by how much)

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

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>