To me, 'slogan' is a funny word with an almost non-Indoeuropean ring to it. Where does it really come from? My thirst for etymological illumination brought me to the steps of the monument that is OED (Oxford English Dictionary):
ad. Gael. sluagh-ghairm, f. sluagh host + gairm cry, shout.
- a. A war-cry or battle cry; spec. one of those formerly employed by Scottish Highlanders or Borderers, or by the native Irish, usually consisting of a personal surname or the name of a gathering-place. b. transf. The distinctive note, phrase, cry, etc. of any person or body of persons.
So, it's Celtic... What I find particularly interesting is that both 'slogan' and 'motto' refer to something that originally was a battle-cry. The etymology of 'motto' may not be all that different. The post-Classical 'muttum' is taken by Lewis & Short to mean 'a mutter', 'a grunt'. Niermeyer gives for 'muctum', (muttum, mutum) the meanings 'grumbling', 'squeak', 'cry'. In essense, a 'muttum' is something that defied comprehension, which is what war cries are all about, I believe.
It is rather strange that 'slogan', it my opinion, is becoming more and more a term for a trademarked catch-phrase, even though the definition found in OED is not that dissimilar from the definition of 'motto' ( hence my Latin motto generator is just as easily a Latin slogan generator:
Originally: a word, sentence, or phrase attached to an impresa or emblematical design to explain or emphasize its significance. Later also: a short sentence or phrase inscribed on an object, expressing a reflection or sentiment considered appropriate to its purpose or destination; a maxim or saying adopted by a person, family, institution, etc., expressing a rule of conduct or philosophy of life.
I guess, the use of 'motto' is simply more widely spread, with more secondary meanings and derivatives. The OED article features a derivative adjective 'mottoless'. As I can gather from the examples, it is a bad quality. One ought to have a motto or a slogan!