(1) makes any difference to baby and As per question two's answers dont think baby is gonna be fussed either way.

(2) if one stimulates baby more than the other . I wouldn't say one is more stimulation that the other at all some babies like certain shapes and colours while the next child may not be fussed

(3) if plastic is better because less germs than soft ??? Am hoping to be able to get bouncer with vibrate mode as well as some sounds. I have spoken to many ppl regrading the vibrating and i have felt the vibrating bit personally i didn't think it vibrated that much it was so soft i could hardly feel it so wouldn't spent extra just on that feature alone. I would choose something that would last more long term like plastic but more like rubber so you can pop it in dishwasher or clean it easily plus something that detaches so when they are bigger you still get the use.

thats just my personal opinion i am a mother of 4 children hope this helps