If it's a 4 (non S) it's a HUGE upgrade. iOS7 barely runs on a vanilla 4, plus you get Siri which I absolutely love.
I had a nexus 5 and a 5s for a while so I can compare the 2. The screen on the N5 is beautiful, no way around that. But iOS is polished to the max in terms of functionality, everything just works where on Android you will constantly be finding these little turds of disfunctionality (is that a word?). For me I needed a phone/personal assistant first, and a gaming/internet browser second so I chose the iphone. But if your needs are more screen centric then I wouldn't blame you for getting the N5.
Another factor to consider is the camera on the ip5s is much nicer, it's nicer in low light, and it's a LOT nicer in terms of catching moving images. The N5 was pretty useless with anything moving, like my toddler, but otherwise it shot great pics, but that was a dealbreaker for me. Also low speaker volume on the N5 wasn't appealing. The hope is that both of these will be addressed with a software update by google.
The software update for the camera began rolling out yesterday and people say the camera is top notch now. The focusing issue is gone now as is the blurring issue.