A MLU will be full of lovely kind relaxed MW where as the maternity ward has doctors and consultants roaming around (there is a theory that doctors and consultants are very quick to suggest interventions when they may not be necessary).
In theory the MLU is a far more relaxed place and good for water births, hypnobirthing, keeping it as relaxed and natural as possible. My closest one is lovely, a lounge to relax in before things get serious or after baby is born, a room that DP can stay in if necessary, your welcome to stay longer after birth while you get used to the new addition to your family.
That said mine is a 20-30 min drive from hospital, there are no doctors, no consultants and you are limited on pain relief options. So pethadine (sp?) is available I think, gas and air but no epidural. If you need help then you need a transfer by ambulance to the nearest hospital.
Some MLU are in a bigger hospital so the transfer is shorter and in a wheel chair or on a bed! I didn't make it to my MLU as they wont admit anyone over 2 weeks past due date. But the hospital was also lovely.
I ended up in special care due to an EMCS and both me and baby needing IV antibiotics so didn't see the maternity ward as we were in a private room.
However in most cases you can if you have a relatively uncomplicated birth in a maternity ward, you can ask to be transferred to a MLU post birth and make the most of the MW looking after you and helping you get used to baby. The MW in special care really helped me to get BF established and were wonderful giving DC cuddles when I needed sleep post op.
At least that is my take on the difference from my experience a year and a half ago.