That depends on the document. You may not need either one, or only one or the other. Bleed is for objects that touch the edge to extend beyond so they don't leave a white line if the trimming is off a little. Typical bleed allowance is .125" or 3mm, but you may want to set it to 0 for inside bleed on a facing pages document that will be folded and stitched after printing (there are a LOT of threads about inside bleed). If none of your content bleeds, you don't need a bleed allowance at all.
Slug is for information you want to appear on the press sheet, but not on the document page. It doesn't get used a lot by most users -- printers marks can be added automatically during export and there aren't a lot of other bits of info you would need. The slug size would depend on what you need to put into it.
You have to ask your printer. He is the only one who knows what you need.
Slug (also called Information) is not used for prining, but for other information.
Bleed is needed, when you have objects—background colours, images, patterns—at the edge of the final paper. You have to extend these objects up to the bleed. Depending on the production workflow of your printer you might export with or without marks and the values vary.