Sorry, where are you getting those flight durations from? OneWorld timetables tells me all SCL-BUE are 2 hours- 2 hrs 5 minutes regardless of the aircraft type and the return is 2 hours 20.
As for aircraft type: I haven't flown this route, but I found LAN really good for looking after status passengers (4 flights, 1 op-up, 1 1st row of Y, 2 in exit row with noone next to me including a longhaul flight) so I'd probably go with the 767, as the J product was better so nicer if you get upgraded! On the 767 I had, which was a 5 hour flight, there was flatbed, whereas the 319 was an inch or 2 more legroom and a table for the middle seat...
EDIT: OH! Argentina no longer practices summer time, Chile still has it - this could account for the discrepancy if the computer system hasn't taken it into account for some flights, or if they have been entered incorrectly?