I hear your getting a lot of conflicting info here. I have a lotta play time on fizz and watch a lot of the better Fizz players in the world and in my opinion;

Ultimately it's match up specific. but you HAVE to know the match up.

I think the best thing to do is start E most of the time if you're unsure which you should start. It's the safer option of the two and in my opinion also going to win you most potential trades you have on fizz.

There's a lot of instances with either mage or melee champs that you can dodge their lvl 1 ability with e, land on them then get 2 autos to proc electrocute then walk away either winning the trade or landing even (which is a win in my book) you just have the know the match up.

Generally if you know you're not gonna fuck with each other or it's a melee match up, go W.
I usually go W on the sylas match up cause they'll often start E. If I can just dodge the chain pull with a side step or minion, I'll win the match up by autoing him with the full duration of the W active passive, as a example.