I have never used the official version. I recently purchased an unofficial one from Gamestop. The guy there claimed it was the official version but I took a good look at it when I got home and saw it had no Nintendo logo on it or anything so he was wrong about that. I tried it out and it works just fine. I set it up with my router and modem and it connects. I don't really have any way to compare speeds or anything of that nature. I should probably run some benchmarks on DSL Reports and see how fast it is compared to my PC. My DSL connection is the cheapest one they offered so it isn't that fast anyway.
There are supposed to be a few USB Lan adapters that will work on the Wii that are supposed to be a lot cheaper than these Wii versions. There was an article I read about those few models awhile back but I forgot the address.