for us the difference are:
Start button is on the bottom and the button is larger
There is a status pannel on the Today screen when tapping on the top of the screen
But I think that 6.5.1 is better (in some programs on 6.5.3 there are no buttons - tested on pocketrar; and some programs have smaller square and it is a Today background on the half of the screen)
And the last time I use WM6.1 because all programs work faster even if the amount of free RAM on 6.5 is grater
Oh, I will be happy if U understood me, my English is not so good as I've imagined...