I've never used neither so this comment is solely based on reviews and my experience with several Motorola devices.
First off the G5 Plus is a budget phone (some will also call it mid-range so let's just say it's not a flagship). So you may lose a bit on general performance (especially with the 2GB version). I'm not sure about the iPhone 6's camera performance in low light but Motorola cameras has never been very good in low light. So if your iPhone 6 is good in that respect, you'll lose a bit there too. Motorola cameras tends to also lose a bit of the color in pictures: people look paler in certain settings. They're not generally known for their cameras.
Otherwise, although I don't own a Moto G5 Plus, Motorola/Lenovo generally have great performing phones on low/mid-range budget phones. So that shouldn't be a problem as such.
Now if you're able to pay a bit more and don't absolutely need to get a Motorola device, you could also go with a OnePlus 3T or a Samsung S7 which are high end phones but that don't cost all that much. Both devices have pretty good cameras.
I personally have an S7 right now which has a great camera but am looking forward to the Moto Z2 Play that should come out in the coming months (I'm a stock Android fan and Motorola are the only devices I can affort
). So when that comes out I'll sell my S7 to buy out my contract and get that new Motorola device (camera performance have never been really important to me).
Hope this helped!