Which is more beneficial, all branches being united as one branch with different missions or multiple branches the way it is now?

I applaud the out of box thinking, but that is basically what we have today. All branches are united under the Department of Defense with three Military Departments (Army, Air Force, Navy) which in turn have under them all the branches of the military (Army, Air Force/Space Force, Navy/Marines).

There are significant training, resourcing and organizing functions that each Military Department* accomplishes for the service(s) assigned to them that would have to be replicated at some other organization if they were to go away.

I can't see any pros in either restructuring to add a stovepipe between the Military Departments and DoD or to eliminate all the Military Departments and have all branches answer directly to DoD. The cons of doing so would be numerous.
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* CRS Primer on Military Departments - https://crsreports.congress.gov/product/pdf/IF/IF10550

It sounds like the same thing just with more steps.

SGT (Join to see), interesting question--and the responses are spot on, IMO. The C2 structure of the Armed Forces certainly impacts how they are organized, trained, and resourced—which are areas that Congress has constitutional authority over. In a scenario that aligns all the services under one command, how the Services provide support to the 11 Combatant Commands should be assessed. This is where the pros and cons can be fleshed out. Combatant Commanders (CCDRs) report directly to the SECDEF and have only one commander above them -- the Commander in Chief. All CCDRs send a demand signal to Congress for how they need to organize, train, and be resourced, which drives the requirements for the entire DOD, including Sub-Unified Combatant Commands and the Services.

-National Security Act of 1947 (amended in 1949)
-Goldwater-Nichols Act in 1986