I have used both as a customer and have them advertised on my websites/blogs.
Sportingbet is the better of the two. The drawback with bwin is you have to pay withdrawal fees so it’s one thing that I always make known to people I refer to them.
As for merging, I doubt that will happen. At the risk of sounding like a conspiracy theorist, I think there is a reason all these bookmakers are teaming up and it’s not to create one singular brand.