Hello! Welcome to the forum!
If you want tangs and triggers or other large fish you will need a 6 foot tank so 150g is the right neighborhood as far as size.
In saltwater the larger the tank the easier it is believe it or not. The extra water volume will allow you to have some leeway as far as water parameters. In a smaller tank if something goes wrong the smaller amount of water in the system will cause things to decline quicker than they would in a larger tank.
I would pass on the damsel fish though, nasty little buggers, they will take over a tank and make life miserable for any other fish in there.
And yes, in my opinion a sump is superior to a canister filter, especially with a larger tank. On anything 55g or smaller then I would consider either hang on equipment or a canister if a sump will not fit under the tank.
If you have any other questions please feel free to ask away, there are a TON of great people on here that are always willing to help.