Hi,
the disco td5 is lower, and has crossmembers under the gearbox which finds all the rock and boulders a 90 doesnt even see.
the earlier discos have a lower front valance and this will take a beating and you will loose the tow hitch cover plastic.
the facelift versions i think have a better ramp angle.
the td5's will have either anti roll bars or ACE control, so articulation will be limited compared to a defender with no anti-roll bars.
ACE is a royal pain once it starts to leak.
the brakes have a tendancy to seize at the guide pins on the td5, regular maintenence is required to keep them nice. the defender calipers dont seize as often.
the disco will go fast and is comfy, defender is a shed on wheels.
you can get mud tyres for the disco, but they have to be smaller ( rolling radius) than an 90's max about 235/70 r16 whereas the 90 can get 265/85 r16 etc, other wise they will rub on the bodywork.
all the disco td5s have a difflock, the early ones (with traction control) just dont have the locking link on the Hi-Lo gearstick, you can lock them by getting underneath and using a pair of pliers, turn the difflock knob. there are aftermarket actuators and gears levers to allow you to use the difflock.
the disco is nice off road, but the 90 is more fun and will go further.
if the disco has traction control (i think they all do) the you need to have the wheels turing at least 5kph in order fo spin to be noticed my the ecu and for the control to work.
My solution is to have the td5 disco and a 110 for the really muddy stuff.
there are other folk on here who use the td5 disco all the time for all events so whey will have a better idea what it is like in the mud, but i've found it to be not as good as the defender.
cheers