I tried both out for a few weeks before sticking with the Phone 3. I really like the 4a Pro too, but the 3 was just better.
Design is obviously personal preference, and they are very different from that perspective. I did prefer the way the 4a Pro felt in hand, I think slightly thinner and definitely feels built like a flagship.
The Glyph Matrix is present on both, but the 3 won that fight by a long shot for me. Do like that the 4a Pro matrix is bigger, but the dense pixels on the 3 give you way more options as far as what you display there. Both give you the same functionality, but the 3 was easier to customize using your own images/logos/icons.
Both phones were smooth and ran well. The only delay I really noticed on the 4a Pro was when taking pictures- it was slower to process each image.
I don’t think you can go wrong with either, but if you just want the “better” phone, it’s probably going to be the 3.
