From what I've saw on this forum, I would guesstimate that Uniden would be somewhat ahead of GRE/Radioshack in terms of the number of units sold.
Likewise, for Europe, since GRE have not really considered European market in any of their recent scanners apart from the PSR-295, it's fairly obvious that Uniden has captured this market as well. One only needs to look at trade fairs, auction sites, and classified listings to see that there are far more Uniden's than GRE scanners for sale over here.
I started out using a GRE scanner which I found was very good, especially for the weaker signals, but very basic. Once I got a Uniden I found that the overall build quality and design of the scanner was more ergonomic and attractive. No GRE scanner over here has Close Call/Signal Stalker/RF Capture so obviously when Uniden released the Close Call concept in 2004, the market shifted towards that.