So I played both demos. (Had to install a font for some reason). These are the opinions of some guy on the forums, so I recommend a grain of salt be taken as you read the text below.
They're both side battle systems, one is more action oriented while the other is an ATB system. So I'll use "Action System" and "ATB(Active Time Battle) System" to distinguish between them.
The Action system has clunky controls, terrible hit-boxes, and unreliable ally AI. I couldn't even hit an enemy right in front of me 80% of the time with a melee character, ultimately causing the party to be wiped out against relatively weak enemies. I got through all of the battles by using the lady with crossbow, since she's the only who doesn't suffer from such hit-box issues. I don't know if skills have been implemented in this or not, I couldn't use any of them and there were no real instructions as to how they'd work. Some of the characters can't jump, some can... just saying. It has potential, but it's too broken right now to recommend.
If you do decide to pursue the Action system, I recommend removing the possibility of "missing" an enemy that you clearly hit. Accuracy and evasion should definitely depend on the player's ability to control a character, and the enemy's ability to move out of the way of attacks.
As for the ATB system, there's not much to say about it. It works, which obviously puts it above the presented Action system. Also, while there were some interesting skills available in the demo, they were ultimately pointless because each battle could easily be won by repeatably pressing the "attack" button. Competent enemies and necessary skill usage are required so that combat isn't just a boring chore.
I know each system is still in development, however, I can really only provide feedback based what's provided since I have no idea what they may turn out to be in the future.
If you're confident in your ability to execute either combat system well, my vote goes to the Action battle system. If not, ATB is a safer choice (though balance is by no means easy).