As you know, Alt-Print Screen will send the current window to the clipboard. If Grabby is running, Grabby will quickly grab the image and write it to a user configurable directory in the desired format. When operating in Bitmap mode, Grabby grabs the full resolution of the screen so that nothing is lost. There is no compression. Depending on the size and resolution of your monitor, these images can be rather significant.
Bitmaps are much larger but the best quality.
JPEG on the other hand are computationally expensive but have a small file size. Well again this depends on your system. On a system with a very fast disk and a moderate to slow processor, you may actually see the large .BMP files as the speediest choice. However, if your computer is fast and your disk is slow of fragmented, then JPEG will probably be faster. The best test will be one where you grab an image similar in size and 'richness' to the real screen shots you will be taking.