There is a limitation a piggy can do regardless of which brand you go for as they only tweak certain parameters. Some chiptuning add in throttle response module to make the butt feels better other than that the figure they churn out are more or less the same. Of course having said that, some piggy can be really different if they are custom tuned to fit our driving style and conditions. Piggy can easily be removed before sending to AD if you are worried about detection.
ECU flash on the other hand basically enable a tuner to change many parameters and some may even capable of breaking the "lock", making a 116i to be a 118i:yikess: As this a ECU flash, somewhat like a firmware upgrade to your router, it writes directly into the physical ECU unit. Even flashing back to stock map will render the warranty to be void because it's not what map you are on, but more on you have tempered with the unit "illegally" that may potentially cause failure to other components in the car.
Whichever route you choose, there are certain risks involved therefore end of the day it's really your choice. If warranty is not a concern, i'm sure many will go for ECU flash.
My inexperience 1 cent opinion....