d9780
Everything is virtually the same from the OS end. The differences would be hardware-related, like screen refresh rate, wireless charging, etc.
From what I've read on here, others who have the 6a are happy with their phones as well. I don't recall anyone saying their usage has been affected by less memory.
I did a quick search and found this quote: #18662. Seems like a community member, spiral, is okay with their 5a and I think their quick review is pretty objective. Maybe it can be helpful for you.