You mean besides the fact that they are separate breeds, different sizes, and different weights??
Holland Lops are at max 4lbs, sit upright, should look like a lower case n with a head. They are the smallest of the Lop breeds, and tend to be more hyper than Mini Lops.
Mini Lops range between 4.08 and 6.08 lbs. They are a typical compact typed rabbit and should look like half a basket ball with a softball head. They are called "mini" because they are a mini version of the French Lop. They have great personalities, tend to be more laid back than Hollands, and are wonderful mothers.
While on the subject of mothering and litters, Hollands tend to have very small litters. The average being 4. They tend to have more issues with kindling. Breech births have to be pulled usually. Because of the size of the heads on the hollands. You also tend to get more first time mom's who have harder times. They are also dwarfs, which means that you can, and will get peanuts.
Mini Lops have larger litters, my average being about 7. They are well tempered and make wonderful mothers and foster mothers. I have yet to have to pull a baby. Mini Lops are not dwarfs, so no peanuts....
I have raised both breeds, and overall, I prefer Mini Lops.