I have one of each one of which is redundant .... lol
I find them fab for my flat back non withered Section D, think its more important to find the right girth! The only one that stops the roll seems to be a genesis which is elasicated slightly all the way through not just at one end.
They are pretty much the same, apart from the T6 can feel a bit sweaty in the seat area, if you know what I mean - several people I've spoken to experienced the same. The t4 is stickier.