Love both of them! My daughter's middle name is Reed, and we considered Wren as a first name. Never thought of Renn, but I guess I'm a traditionalist in terms of spelling.
So my vote would be for Reed. Although, having said that, I think "Wren Reed [Surname]" is an amazing name rhythmically, but it does depend on your surname.
ETA: LOL, just read your other thread and realised you mean these as a boy's name. I would personally go for Renn Jameson for a boy, as to me, Reed is a girl's name.