Discovery 2 vs Defender 110 cabin size
Cargo areas to back of second row..
D2 - 40.5 ft3. (1147L)
D110 - 56.5 ft3 (1600L)
D3/4 - 44.5 ft3. (1260L)
More than just cargo space to consider
Tombie has provided you the volumes and I think you have got a fairly good spread of comments about the vehicles you asked about. There is probably a bit more to consider than just cargo volume.
I have both vehicles, no children but do have a dog, therefore partially qualified to comment. Both mine are diesel one 300Tdi the other Td5.
The Td5 is the better engine, if you can rule that out by getting a Td5 Defender then that is one less difference to consider. One thing that may sway it is the practicality of a carpeted vehicle versus the utalitarian rubber mats of a defender. I try to towel the dogs down before letting them jump in the back seat of either vehicle. Unless you have a cargo barrier you will struggle to keep the dogs in the cargo space. The effectiveness of the a/c will probably rule out having the dogs in the defender behind a cargo barrier on a hot day. Unless you have something smaller than a fox terrier it is unlikely you will be able to fit much cargo and a dog in the cargo space of a D2. I think it is one or the other and not both dog and cargo in the D2 cargo space. Both vehicles rear seats are not that comfortable, D2 seat is better but the seating position is not that great. The seating is more upright which allows for less leg room but both vehicles have an adult’s eyeline above the top of the door windows which means looking out the sid s is no comfortable. Looking forward from the back seats is also compromised as your eyeline is level with the top of the windscreen. Neither vehicle is great for adults or grown children in the back seats.
I think if you are after comfort for two adults and pre teenage children and a dog then the D2 is pick. If cargo space is more important then take the Defender. As soon as your kids get into their mid to late teenage years neither vehicle will be great. The D3 and D4 are likely to be better suited to four adults or children of adult height. But they are different vehicles which you may want to go and have a look at before you decide.