The G70's vocal processor is very good but I would imagine that when the salesman says even if you can't sing it will make you sound good is a bit of a sales pitch m8. I'd imagine that you must have some vocal talent and a bit of know how when using the processor to get the best results. The Korgs top end arrangers also have a superb vocal unit in them as does the Tyros 2 when it's set up correctly.
If you want 76 keys then either the Roland or Korg should be great. I can't sing for the life of me but if a salesman said to me this arranger will make your vocals sound great I'd imagine he is only after his sales commission fee lol Ask the salesman if he can sing, if he says NO then ask him to sing into the G70/PA1x pro and see if his voice suddenly sounds great.