Hi,
That's a tough call!
Depends what you will be using it for. I have watched a few u tube tests/ benchmarking against more expensive phones that may be of use to you. To me they seemed mostly to be subjective and in 'real life' situations it would be hard to notice much speed difference for most tasks, although if you use memory hungry latest games it may be more of an issue.
I've not had my Moto G long but not yet seen anything that is what you would call slow - far from it in fact. The one thing I'd say is look at the camera? If you take lots of pics it may be an issue as it is only 'ok' not 'great'. Other than that it is very hard to justify spending more on a phone that, like all phones, will be out of date very soon anyway! I aim to keep it for 2 years or so and then get the new equivalent. (I can use the savings made from not having a high end phone to pay for the next new one!)
Try adding up the financial cost over a year or however long you expect to keep it and see what you could buy with the difference?!