The differences between all of these will honestly be pretty minimal unless you've spent a lot of time with varying audio gear and know specifically what to listen for (an aspect which can equally increase or decrease your enjoyment of the music, so enter at your own risk). All the iBasso products cover a particular niche...
T3/T4: ultra portable amp, no dac, cheap
D2: tiny portable dac/amp, relatively cheap
D2+: upgrade over D2 which is no longer available
D4: bigger dac/amp, 9V batteries, more headroom (ie: volume/power)
D10: bigger yet dac/amp, optical/coax
D12: big brother to D10, dual dac
P4: really powerful amp, no dac
...and so on and so on. Pick the one within your budget and requirements, then stop worrying over details and enjoy the music.