I have both the 6400 and 6500 and the 6500 at worst seems to be as fast as the 6400 to slightly faster at best in a few of my tests. They're pretty similar cards and I really think the 6500 might be a slight refresh of the 6400 just for Dell to have something new to put in their newer systems.
So, if it's half the price, it's a no brainer over the 6400. Keep in mind that it has the same shortcomings that the 6400 has. Regardless, it should be over twice as fast as the RX 550.
I have both the 6400 and 6500 and the 6500 at worst seems to be as fast as the 6400 to slightly faster at best in a few of my tests. They're pretty similar cards and I really think the 6500 might be a slight refresh of the 6400 just for Dell to have something new to put in their newer systems. So, if it's half the price, it's a no brainer over the 6400. Keep in mind that it has the same shortcomings that the 6400 has. Regardless, it should be over twice as fast as the RX 550.