The Glonass is nice but this one is specifically made to go in a vehicle, has external harness wiring, and there's something about "strong magnetic" in a lot of the descriptions I read (even though I can't quite figure out what is magnetic from the descriptions). I also think the TK109 has a better battery since it's not just a backup.
I still like the TK109B better for an external, detachable solution. I've been testing the TK109B for about a month now and it just plain works. Very accurate and has not failed to report it's location in probably 100 tests so Glonass is nice to have, but I don't see it as a necessity for this.
Best of all, I can buy just one.