Implicit conversion uses a better plan
Why is that?
Sometimes, the change is simply down to the different starting point (initial shape of the tree) that resulted from the change in syntax. Sometimes, it is because a seemingly innocuous change caused the optimizer to explore the available options in a different order.
The optimizer hardly ever performs an exhaustive analysis (within the scope of things it knows how to do), so it's not really all that surprising that small changes to the inputs can result in a different path being taken through the optimizer code, with a correspondingly different result.
Generally speaking, implicit conversions are bad news for cardinality estimates, and consequently for plan quality. There is a large element of luck involved in that adding the convert gave a lower cost and faster performing plan. This will not often be true, and it's certainly not something I would recommend you try, or rely on, routinely.