Filter Sponges

1) The Juwel media is no better but the sponges are cut to be an exact fit, which others might not be. If you buy them bigger and cut them down be sure to keep an old Juwel one as a template. The sponges need to be a snug fit because a gap will make the water take the easier option of going round the edge rather than through the sponge, defeating the whole object of a filter.

But the only part of the Juwel filter that needs replacing is the white pad. Before I removed the built in filter in my tank, I found the white pad got dirty very quickly and disintegrated on the second wash. Since this has to be changed so often, get a roll of filter wool (pond stuff works out cheaper) and cut it to shape.
In almost every other filter, the white pads go last in the water flow and are used for 'polishing' the water, ie removing tiny particles that pass through the rest of the media. In the Juwel filter the white pad is first; it is there to catch the big bits of debris so they don't get into the rest of the media. If it is changed frequently, it won't have time to grow many bacteria before it is thrown away.

The other filter media doesn't need replacing. I had coarse and fine blue sponges which should last for years; a green nitrate sponge which doesn't actually remove nitrate, so treat it like a blue sponge; a black carbon sponge which I never used; and the white pads which I changed every 2 weeks. My tank is pre-cirax, but that only needs changing when it starts to crumble.

2) You don't need a carbon filter. The use of carbon dates from waaaaay back to when the only filters in use were a box full of carbon granules with a layer of filter wool on top to keep it from falling out, all driven by an air pump. Water changes were thought to be bad; they were done a couple of times a year. The tank water got yellow over the months and the carbon removed the yellow colour. No-one knew about filter bacteria back then.
Now that we have our modern filters with their bio-media and know we need to do weekly water changes, the only use for carbon is to remove the brown colour coming out of bogwood (though that's for our benefit, the fish will like it), to remove medication after treatment has finished and to remove any odd smell in the tank.
I haven't used carbon since I found out about this 10 years ago, and my fish are fine. Keep a carbon sponge or tub of carbon granules in the cupboard in case you ever have to medicate but don't bother using it in the filter full time.
Since the carbon sponge in the Juwel filter is only thin, just leave a gap where it fits so you can put one in if needed.

3) I turn them off, but then I clean my filter media during a water change so I have some old tank water ready. With internal filters, they have to be turned off during a water change as they get damaged if the impeller runs out of the water. I think they've changed the design of the Juwel filter since I got mine - I had no option but turn it off as I had to take the pump off to get to the media.
As for turning it off to change the media - if you really do need to change the media, turn it off then as well. But the blue and green sponges and cirax don't need changing. Ignore what Juwel says. If you never buy new media, they don't make as much money. That's the only reason they tell you to change it.