Neither. I used a volo chip. It did nothing. Problem is that the computers are too smart for "piggy back" systems. You won't see the gains you really want.
What you need to do is alter the sensors individually. Yes it may be illegal in some states. Check with your state and check with yourself.
You need to tweak both downstream and upstream o2 sensors to get them to read a rich condition to lean out your car. You can use an EFIE or just buy some potentiometers.
Then, in order to keep your computer from catching on to what you've done, you need to also tweak the MAP or MAF sensor in your intake. Depending on which type your car has. This will ensure that the ECU does not see that there have been changes and the current amount of fuel in your system is consistent even with the introduction of HHO gas into the engine.
I had a link to a great site that showed you what you need to do to keep the HHO gains. But I can't find it. I will did it up if you're interested.
I purchased a Volo HHO edition chip and am still testing it out, but so far have been satisfied with how it performs on my 2004 Ford Explorer V8. From what I've heard results can really very from make, to model, to year of the vehicle so yes if you're looking to pull some larger MPG gains this is probably not for you. If you're just staring out and don't wish to purchase a pyrometer (exhaust temperature gauge) to ensure you don't melt a piston this may be a good option.
I'm running a pyrometer with my setup and the Volo chip with the HHO cell keeps the exhaust temp cooler than what it ran when it was factory. (without modifications)
Summary: A Volo Chip - HHO Edition may be a good fit for you if you are just starting out, however there are many factors, make, model year, HHO production amount, etc.