Neither. She's not a traitor, and never will be. Even if Gaspard becomes emperor, Celene never betrayed Orlais. And she's certainly not a savior considering that Orlais got embroiled in civil war on her watch.
What she IS is a horrible empress. Let's start with her first mistake. She dallied with a female elf since her teen years, all the while refusing to marry and have children to prevent a succession crisis. As an empress, it was a duty she blatantly ignored. Using her single status as a bargaining chip in her political life is a worthy strategy. But overuse of that strategy can create an unnecessary succession crisis that can lead to war, which is why so many nobles where willing to side with Gaspard. If Celene was a married woman with few children, her rule would have been much more solidified, and she could have carried on with Briala to her heart's content.
The next mistake was her refusal of Gaspard's marriage proposal. She KNEW he was offering her a chance to avoid war. She knew he was serious, what kind of man he was. And she refused because she just couldn't let Briala go. As a empress, this was a selfish, idiotic decision that has brought and enormous death and destruction on the people that Celene swore to protect and defend as their ruler. Had she just swallowed her pride and her attachment to Briala, countless innocent lives would have been spared.
The last major mistake? Taking Gaspard's bait and sending troops to crush the elven rebellion. She underestimated Gaspard's political and military skill, and it cost her her best, most loyal troops. The fact that Gaspard tried to kill her should have indicated to her what Gaspard was willing to do. I'm not saying she should have seen Gaspard's every move. Just that she should have thought it through. 'Why is Gaspard trying to provoke me?' 'What does he want/expect me to do?' Had she asked herself these questions before going off to slaughter the elves, her men wouldn't have had to pay such a high price for a political mistake.
However, all of these mistakes aren't the reason I hate Celene. It's that she made them after Briala's parents and the rest of her household paid such a high price to make her empress. She has them killed, but doesn't marry and produce an heir. She has them killed, but won't marry Gaspard to prevent civil war. She has them killed, but reacts to Gaspard's bait so foolishly. They pay the price for her rule with their lives, and she bungles it all up so spectacularly.
She and Lady Mantillion believed she'd be better for Orlais than the aged Florian. How, exactly, has she been better?