>>Put the FROM Menu INNER ... or not put it is the same on the execute plan
>>
>>The true difference is on FULL JOIN instead the UNION.
>
>Ah, I didn't pay attention to the FULL JOIN at first. So, basically, it is just a choice on how we write this SQL command.

In SQL Server you want to test proposed solutions as you don't know in advance (unless it's obvious) which solution will perform better.

The simplest (although not necessary accurate) way to test is to put both statements in 1 batch and run with execution plan on and then see percentage difference.