8 Speed - 9 Speed

Hi All
My son's 8 speed mech has had it, so going to get a Shimano Deore 9 speed which I have been told will work with 8 speed as well.
How much difference is there between 8 & 9 speed, just thinking if I'm going to get a 9 speed mech should I convert his bike to 9 speed, it will cost around another £35 ish pounds for the cassette, shifter and I will put a new chain on as well, but if there isn't going to be much difference between 8 & 9 speed ??
My son's 8 speed mech has had it, so going to get a Shimano Deore 9 speed which I have been told will work with 8 speed as well.
How much difference is there between 8 & 9 speed, just thinking if I'm going to get a 9 speed mech should I convert his bike to 9 speed, it will cost around another £35 ish pounds for the cassette, shifter and I will put a new chain on as well, but if there isn't going to be much difference between 8 & 9 speed ??
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Posts
Just fit the mech and let him ride the bike.
Or you are paying for shifters (brakes?) chain,cassette, cables for not much.
Parktools :?:SheldonBrown
That sounds the sort of advise I was looking for