When you look at reviews/impressions, you have to correlate it with a date
At the time when I was talking about AMP3, there was only stock AMP1 and new AMP2. Now, AMP2 is replaced with AMP5, your single-ended 3.5mm option, and AMP3 still remains as your 2.5mm balanced option for HO and LO. To me, sonically AMP4 is the best, but as it was stated in some of the previous posts, it was a limited run and they no longer manufacture it.
At the end of the day, it's not about which one is better, but which one pairs up better with your headphones and also if your cables are 3.5mm or 2.5mm terminated. I mean, you can always go 2.5mm to 3.5mm with an adapter (CA01 is very good), but some people have all their cables terminated with 3.5mm and it will be too much to invest into all new 2.5mm cables. Btw, I went into a detailed sound difference between amp modules
here. For me personally, I have AMP4 "permanently" on now. If you take that option away and I have a choice between AMP3 and AMP5, based on pair up synergy I prefer AMP3 with iems in my listening rotation.