tweecer, You can write a program without shutting the car off! You have a choice of up to 4 programs that will switch on the fly, GUI software is ok.
SCT, you will have to stop the vehicle for a couple minutes to reburn
just as much is possible with either product until you go to tweecer RT, then you may see all data coming into the EEC. Datalogging makes things really easy to solve compared to shooting in the dark. In order to datalog with SCT you must buy separate hardware which will cost you another 200-800us depending on what datalogger/scanner you choose.
Tweecer is better IMHO because you are not taking a chance messing with the internal rom and you have a switched on the fly 4 programs! or turn it off while still in the J3 port.