So do you think the 2017 rMB Kaby Lake M3 is faster than 2016 rMB Skylake M5?
Maybe, maybe not. If it's faster, so what? If it's slower, so what? Whatever the benchmark numbers are, the real question is what are you going to do with it?
If all you do is just general surfing, emailing, youtube, and probably some database accounting, you're better stick with what you already have. If speed/performance is a concern to you, you're looking at the wrong product line. At best, it's nice to know the newer machine might be faster, but after about 2 weeks of bragging, you won't even care.
I still have the MB 2015 version, it's still perfect for me. For my use, it's not slow at all. But since the new machines came out, and if you're within your return window, you can decide yourself if you want to keep the better spec previous gen or newer MB. Other than a slight CPU speed bump, everything stayed the same. Unless Apple offers MB 16GB RAM 1TB SSD at $1600, it's essentially the same thing I bought 2 years ago. The display is still the same, not the more vibrant P3 that's in the MBP. Same 480p camera.
If it was my choice, I would keep what you have. The CPU difference won't matter, but the SSD will. You already got the 512GB at a discount, meaning more actual capacity to store more files. If your computing needs are basic, you will not see any benefit from the slight CPU increase, if there is any.