The only bar I researched is the brand "R" for the roof, the biggest difference besides the fact you can get it in 54 inch, wider spread and good looks, (if you care about such things, which we are not supposed to when it comes to lights according to some,) is the Lumens. 50 inch roofmount strait 23000 Lumens, 54 inch roofmount RDS 40500 lumens.
Hmm so 4 more inches is nearly double the output? dont you mean 30500?