I don't think you can go wrong with either of those guys. But personally... I keep our young left hander.
Niese has all the makings of a front line starter. He took a big step forward this past season in terms of durability and performance. I don't think most fans know what we have in Niese, but I'm pretty sure most GMs in the league do.
Upton is young and athletic and he's shown some pop. Again, I don't think you can go wrong with either, but if there was a knock on Upton he's shown inconsistency at the plate and he's near the top of the league every year in errors committed.