Its really a personal thing going to all and going to none is a decision you need to make on where your business is, what your objectives are and of course cost vs return. In terms of which one to go to – Hard to say as again depends on objectives, I would base it on
who is exhibiting?
Cost of going?
Who is speaking?
Who else is going?
Or in fact which city do you want to visit 
In terms of major differnces
London – The biggest attended event and very busy
Dublin – Also has B2B section so big expo room – oh and Calvin Ayre’s party
Sept (location TBC) – Big but also a little more relaxed
There is lots more differences but message me if you have any specific questions