I have a white brothers exhaust that came with my 400ex when i got it and i was wondering if i should sell it and get a lexx MXe or just keep it.
They are both slip ons.

 

Keep it. The white bro is a good pipe, use to be the best pipe for the ex. They dont make them any more. Lexx mxe is a ok pipe but not well built.

 

What about hmf?
Is it any better?

 

First I would check the header. I have not seen a white bro. slip on on a stock header, you might have a full exhaust.
I feel a slip on's are all the same, besides quality of construction and tune ability. If you did a test and had 3 slip ons all set up the same you would not notice a real difference between the 3.