There is a vast difference in this aspect; a []SomeStruct makes two allocations (one for the control structures, one for the data), which might be on the stack or not. Conversely an ArrayList<SomeClass> makes a linear number of allocations; each object in it is separately allocated.

As you say, value types are coming soon, but they don't exist now, so right now there is a lot of difference (and in practice will be for a long time, because not everything is going to migrate to value types immediately).