The Apex Power FC is a full stand alone management system. It will give you full control over everything and is in the top tier of tuning options. The one thing that I don't like about the system is that there is no knock safety measures. This is something you might not be able to tune on your own, but the software is out there if you want to try.
A reflashing is going to give you full fuel and timing control, raise the rev limit, injector scaling, etc. It is inexpensive now thanks to openecu and a $90 cable, and more user friendly;
The easiest and safest to use out of all of these the the S-AFC II. It's just a piggyback, controls fuel very well, but you have no direct timing control, and no bell or wistles. It's crude, it's cheap, but it works. Here's a GT30 with 660cc's, stock fuel and timing tables, making 400whp+/350+tq on 91 octane(!);
What are you looking to do with the GT35? How comfortable are you with tuning?