@xraptorx about your questions from a very happy 50l user.
1- Well the 50 l it´s a 25 kilos piece of equipment, but for it capabliities a very compact and easy to handle equipment of course you can beat the small footprint of the 20L unit.
2- I wash in the bathroom, I don´t put the unit in the bathtub (it barely fits in mine anyway) I just take the showerhead and clean it inside with a soft sponge rinse it and dump the dirty water in the bathtub and repeat as necessary. As dinnerstick said both the 20l and the 50l have sharp edges of stell (only in the legs actually) and they can scratch you enamel.
3. It depends how strong you are

whit the short malt pipe no problem at all, the big malt tube its heavier but managable, I mostly brew 6 gallon batches (and I brew every week and two times a week if I have the time) if I started to make all 12 gallon batches I´ll make a pulley or something for it. Brewing outside: did it only once and I prefer indoors, the unit is very silent and I have an spare room for it.
4- You can make make the same beers on the two systems, the 50l has two pumps and the heating element is twice the size of the 20l that gives you more power for recirculation when brewing batches with the short malt pipe, but I´m not sure is there is any difference in the performance of both units.
5- I mostly brew indoors and here its very warm most of the time so very little help with that.
If you live in an apartment with no spare room or guest room that you can use like your "brewing shelter" I would go for the 20l don´t be afraid of not been able to brew bigger beers with the 20l I´m sure and certain that it´s possible.
But as I said I´m a happy 50l user so my choice was the 50l even knowing that I would not brew 12 gallon batches very often.
And I´m sure that when your wife see that the braumesiter is a silent machine and the smell of malts and hops it´s great she will not oppose to you brewing in the kitchen.