types of media
1 din HU can only load 1 type of media wither CD or tape or MD
2 din can load 2 type of medie

well depends on HU lar and usually those that can load 2 media dun have a TFT screen

tft screen
1 din screen higher changes of spoiling as it has a mechanism to move the screen
2 din screen lower chances of spioling as the screen is always open

also it will depend on the HU as some 2 din have moveable parts also

security
1 din will come with removable face panel (dunno if it applies to those with TFT screens)
2 din so far i have not see any with removable face panel

note: becafeful when buying HU which have motorised face panes and such cause they may spoil like what happen to me

i driving toyota also stock 2din HU
2nd slot there you can put other things also if you want

signal processor, EQ, monitor,
of put a wooden panel there cut hole for turbo timer or any meters