You will often see the larger size cheaper than the smaller, this is because the larger size is less "useful".

The larger size is OK for British Airways, the smaller size is OK for Lufthansa, so if you're going to travelling on different airlines, I would definitely go for the smaller size, but bear in mind that even that is too big for some airlines (too wide for AA, for example).

As Till says, Lufthansa is a stickler for their weight limit (9kg?) but BA have a limit which I think is 23kg (but you must be able to lift it unaided).

Till also mentions the gain in packing volume between the 2- and 4- wheel cases; Rimowa's figures for are that the smaller size in the 2-wheel version gives you 35 litres of space, the 4-wheel version comes in at 30 litres, if that 5 litre difference is useful to you.