I have found that printers with frames that are made of 80-20 aluminum can become "loose" after a while. If you have a lot of long prints, you would be surprised how much zig-zag motion that transmit to the frame. This is why I avoid Plexiglass frames and metal frames that are bolted together. I have built a lot of Prusa i3s and the solid frames always work out well for us. We use three of them all the time in our "printer farm".