depends on what you need...Standalones offer a wider resolution of control, and fully take over the stock ECU's fuel parameters and triggering...A piggyback system does not offer quite as much control or accuracy...but depending on what you need done, it may offer more than enough control...and piggy's are cheaper...
Try a search on this if you are interested...there are tons of threads related to it, be careful though...some are filled with bulls*** and flaming...I would check out the EMS forum, that would probably be the best place to start...