Fundamentally, query performance is tied to execution plans, so whether its tweaked in SSMS or by these tools, its still a race to get the best execution plan, including the best performance on each operator in the plan.

SQL Server automatic tuningDatabase Engine Tuning Advisor (DTA)When compared to the use of SSMS the only difference I found was how the execution plans are tweaked. Fundamentally, query performance is tied to execution plans, so whether its tweaked in SSMS or by these tools, its still a race to get the best execution plan, including the best performance on each operator in the plan. However, the tools attack this challenge differently.With the automatic tuning feature, SQL Server monitors the query performance and automatically applies corrections to the execution plan to make it better. Sometimes it applies bad adjustments and it will realize that and automatically revert or find a new plan. It works at the plan level, it doesn’t modify the query, it just finds better ways to execute it. On Azure it can go a step further by automatically adding and removing indexes to improve the plan.DTA is a process you manually start from the SSMS menu or at the command line. It does a fairly in-depth analysis using a server workload session; either one you saved in the past or live activity. The tool gives recommendations similar to what the automatic tuner would be doing, but it gets a bit more advanced. For example, suggesting the use of columnstore indexes and partitioning. So, this tool can suggest changes to the physical database design. This is probably a fairly drastic approach to getting better performance, but I think its targeted at very large databases such as those in data warehouses. So, the DTA will not only get a better overall plan, but will get each part of the plan to perform better due to the physical adjustments it recommends.In summary, the Automatic Tuning feature will get the best plan without user intervention. DTA will do some in-depth analysis to give the best database design. Using SSMS we can do all the things the tools do, we even have the additional option of changing the SQL code. However, using SSMS could be quite labor intensive.