I like Glen a lot, and I definitely prefer this spelling. I think it will come back into style eventually, but it's too soon. The most common age of a man named Glen in the U.S. is 53, and the most common age of a man named Glenn in the U.S. is 48. [Data from Wolfram Alpha, for the year 2018.] So I'd give it 20 or 30 years before it is popular for babies again. And I can also imagine it going to the girls (unfortunately).
I didn't like this name at first, but it's growing on me. I certainly like this more than the other spelling, which makes the "n" seem rather drawn out.