Machineability is better for aluminum.
Titanium adds a LOT of cost for what's really no benefit on a phone.
Stainless steel would probably be a better choice than aluminum though. Sure, it's heavier, but it's stronger and cheaper, and I REALLY don't care how much my phone weighs, it's never going to be heavy enough that it's a problem even if they make it twice as thick so they can put in a LOT bigger battery.
Stainless steel was the one thing about my 13 pro that I hated the most. I hated the shiny finish and I hated the weight. I do care about phone weight...the lighter, the better.
..and my next phone won't be a pro unless the weight goes down.