What you want is downsampling, not cropping. You get 38Mp which you want to downsample to about 5-8mp for best results, the 38 Mp gets interesting when in need of massive digital zoom, but don't expect great photos when that is being used (it will still look grainy/noisy as hell when in 100% mode), also instead of letting the phone firmware handle the downsampling you can save the 38Mp picture and use Lightroom or other software instead for that, eitherway the resulting picture 5Mp picture will always outperform the 5Mp picture of a 8Mp Phone.
The Sensor is 41Mp but it's only using 38Mp (7152 x 5368 pixels) for 4:3, and 34Mp (7728x4354) for 16:9, that is if it's really the same sensor as on the PureView 808, but hopefully the new Lumia also comes with OIS and low light improvements, which would make the new Lumia really be the phone if you want one that does the best images currently possible on such a device.