I went with the BMW tender
I have the BMW tender. I understand it's identical to the Deltran battery tender for gel batteries. BTW, I have an '04 K RS. I went with BMW because the difference in price was minor, maybe $10, and I prefer to give my dealer business if/when they are competetive. The price was ~$75.
If you go with the Deltran make sure to get the one for gel batteries and make sure you get the BMW connection pig tail. It was the extra cost of this pig tail that made the BMW tender price competitive.
I don't have part numbers with me as I write this but I can get that later if it's important.
It works great.