You might want to look into what parts are on your bike currently and do some comparisons in addition to the frames. The frame is probably your most obvious choice (a few pounds there) - but it may also change your geometry and ride - gamble.
Some of the other ways to shave weight:
1.) Go Tubeless - tubes, especially with slime, can add 1-2lbs (at least) to a bike.
2.) Rims/Wheels - a lot of stock rims/wheels are HEAVY - swapping out to a better pair will help drop weight (and you can use the old pair for different (street?) tire treads.
3.) Check your seat, seat post, stem and handlebars - the weight savings on these trade-outs (for far less than a frame) can add up
4.) It starts to get pricey... but cranks, pedals, etc can shave pounds as well - but there's some shop time (unless you can do it yourself) - at this point I'd be looking at a new frame (or new bike altogether).
You might also want to look at other frames - my Giant Trance X0 is around 27lbs (upgraded to XT crank for strength/weight). The risk here is compatibility and frame geometry (gamble).
Best of luck in the hunt...