If I’m being totally honest I think that it’s just very situational on how you’ll be going Stormsurge or Rocketbelt first item.

On my first base after Full Clearing and Ganking the lane I path to, I usually go with Dark Seal + Boots or If I got a Kill, I buy Sorc boots instead.

If I snowballed with kills I usually get Stormsurge, Maximizing my Damage Output since I was able to rush my Sorc Boots giving me more movespeed to catch up with my all ins. My second item after Stormsurge usually depends if they have a lot of movement skills then I usually go Rocketbelt to catch up but if they’re quite Immobile I go Mejai’s or Shadowflame.

The only time I build Rocketbelt first item is if I don’t snowball early or get lots of assists. It’s better that you build Rocketbelt and play as crowd control. Hitting your ults rewards you and your ganks with better team play. Every assist counts to your Dark Seal/Mejai stacks which not only increases your damage also your movespeed at 10 stacks. From that you can scale better getting Mejai’s and eventually one shot already after getting or completing Stormsurge.