Piggy back will be fine for most people on this forum. You final hp goal, injector size, true timing control, and low rpm/idle performance are areas that will decide for you if you need a total stand alone system or piggy back. Also affordability.
If you are using 600cc or bigger injectors a piggy back may not idle worth a crap because the stock ecu will not allow a low enough duty cycle and it will be pig rich. I believe the oem lowest duty is 3% and my aem with 720s duty is 1% and still a little rich. If i am correct most piggy backs dont send the injector duty signal to the injector but the oem ecu does and it does have a maximum and minimum duty cycle. Oh and if you say just lower the fuel pressure to help idle you can but you just effectively lowered the size of your injectors and should have bought smaller ones. Injector size or flow is a function of FP. Also, O2 feedback control is great. The wideband readings will tell the aem to increase fuel or decrease fuel to achieve a user set o2 target. really great feature for hwy cruise and helps in tuning. Not many piggybacks have that feature.
How many piggy backs have true timing control? some i hear can do what 2* of timing. There is a lot of power to be had with timing. It is about as important as boost control/pressure on hp performance. Also there is a lot of safety control in timing. Got knock control too? Very nice features of stand alone. Of course timing is a easy way to blow an engine. Will the oem pull enough timing at high boost?
These two areas will make a difference in cruise and idle performance and around town driving. Chris's aem had a improper originally installed resistor that made his rev limit only hit 5800 rpm. AEM put the proper resistor in and it works His was one of the first built and probably others did not have this problem. Mine did have the 3800 rpm stumble but i did find a ignitor and dwell that works fine. Others never had this problem. I think it may be the cps and the way it is installed or set and the ignotor may contribute to this problem. I fixed the a/c on idle up initialization by wiring in a ground signal directly from the a/c clutch to the aem. My idle is perfect 90% of the time. It still may die after long hot drives when i put in the clutch but i am narrowing down that problem and it is seldomly seen now.
AEM has total timing and knock control. You can monitor and log all areas of engine function, compare maps, different ways of boost control. But with total control comes more risk as you can program the aem to blow your engine up.
timing control and logging and being able to display numerous parameters is why i bought aem. It has taken a while to get thru the learning curve and i am still learning. I am not an expert at all. Tuning WOT with aem or piggy back is easy compared to tuning drivability. My drivability around town and hwy is great as i interpret it. I still need to have some friends drive mine around for second opinions. I am working on a leaning problem with small steady acceleration. When slowly accelerating from 60 to 80 mph the car requires a slow increase in fuel of + 10% or more to stay at 14 a/f. this is under about 1 to 2 psi accel. I dont know where or what controls this.
Any of these options will require tuning. The closer to stock or more dependent on the OEM ECU you are the cheaper the tuning and or equipment will be. You will have stock like drivability of course. I like the drivability of my aem now. I would think the aem would cost for the equipment over piggy backs amounts to be between $600 to $1000. Thats not that much for increase in features. If you are staying below 400 to 450 rwhp piggy backs will do that. A good testimate to our engines and oem ecus. But for 450 rwhp and up do you want to cheap out on good tuning? That is where the engine blows up. Usually not part failure due to exceeding limits but flaws in tuning.
It is possible to get started in piggyback and as you go farther and farther sell it all and go aem. All those gcc and vpc stuff that is 20 year old technology is not worth the money now. Yes it works good with our ecus because that too is 20 year old tech and it can get the job done but it is just not modern. Many mk4 guys scrapped that and went aem. Ten years ago they probably just about spent as much money on the vpc gcc stuff as an aem. I dont think the gcc etc does timing either.
Lots of electronics out there now to tune these cars. If you dont have a high hp goal piggy backs are just fine. Dont have the cash then most stand alones will not be obtainable. Lots of ways to get the job done.