Maybe I'm guilty of misinterpretation, but I am sensing some T'Pring hate here - which is a shame because I really like her. Don't get me wrong: I adore Spock and don't understand why anybody would prefer Stonn. But once I get past that...if she really didn't care for Spock, how many other options did she have?
We are left with the impression that she didn't have many. She could (1) Go through with the marriage ceremony with Spock, (2) She could have Spock and Stonn fight for her (which Stonn implies was the original plan, if I am remembering this episode correctly) and risk losing Stonn, the man she wanted, or (3) She could try something off the wall.
She went with #3. I kind of admire her moxie, and it seems to me that Spock did as well. Stonn seemed a bit out of his depth, though.