I’m having trouble deciding between two options for Multiclass

I’m having trouble deciding between two options for Multiclass

For clarity, I’m going to be making a Warforged Artillerist Cowboy who uses a hunting rifle to attack enemies from really long distances. I’ve already decided to get Spell Sniper and Sharpshooter for future feats. 

Now comes the issue of deciding my second class. Should I choose a Rogue Assassin or should I choose a Ranger Hunter?

I am not really a huge fan of Rangers but it depends, are you going for more a sneak build? If so, I would go for Assassin. If you aren't looking to go for a Sneaky build go for the Ranger Hunter.

I’m thinking of being sneaky at the beginning of combat, but once my party rushes in to attack then I would adopt a more aggressive play style 

Being a sniper is probably fun and amazing, but I feel like that would make the Artillerist’s turrets useless

I think you should go with the Ranger if you are only going to sneak at the beginning of combat and then be more aggressive.

Personally the ranger hunter sounds like it fits your concept a lot better.  Though I don’t think either of the extra damage from sneak attack for the hunter ability applies to spell sniper.  

Gloomstalker Ranger might be a better fit for what you are going for.