I'm a Benchmade fan, but the 912 has two things against it in my book: assisted opening and liner-lock. I am not enamored with AO and I don't recommend liner-locks to anyone anymore. Tantos are a give and take, so if you like 'em, thats fine.
As to the Rift, I really like the Axis-lock. But, for some reason, the blade shape of the Rift does nothing for me. I have a few knives with similar blade shapes and am not enamored. Second runner-up to me.
I'd go Manix-2. The blade shape seems pretty good, as does the handle. The Ball-Bearing lock seems to be pretty good (I have been using a P'Kal for a week - yeah, not very long - and it is working well so far). I don't like the "finger choil" as it takes up measured blade realistate.
If you get a chance to handle these, please do so. It's between the Rift and Manix IMO.