All three of mine have picked a specific corner of their hutches to use as their bathroom, so it makes spot cleaning them a lot easier and faster!
I haven't tried one of the littler trays though, the only pet shop I've seen them in is Pets at Home, and to be perfectly honest I don't think it's worth the money for what it is. They also look very big, and I'd be worried that it would cut down on the space my piggies have for running about their cage.