This is a questions I've asked (and been asked) numerous times, so hopefully I can help you.
Concerning piercings, common con's include risk of infection, the pain involved during/after the piercing, some workplaces frown upon multiple or obvious piercings (such as facial or tongue piercings, or gauges).
Pro's include that many piercings are considered attractive, the jewelry worn in piercings is usually fun, and (depending on the placement of said piercings) pleasure can be heightened.
For tattoo's, they have a certain social stigma against them, especially depending on what your tattoo represents (not that this should necessarily stop you, just something to consider.). Also they are permanent and are very hard, painful, and expensive to remove.
Also, unless you have a job that allows visible tattoos, its also a good idea to keep them below the collar and above the cuff of a shirt (same goes for ladies wearing skirts).
The pro's are that you end up with a beautiful work of art that travels with you, is a conversation starter, and can remind you of an important event/or reason why you got the tattoo.
The same risks of infection apply to tattoos as to piercings. For either one you want to make sure that the shop you're visiting is clean and sterilizes their equipment properly.
Hope that helps!