I wouldnt really say it's a good or bad idea but you wont gain much from it. They're essentially the same bike just two different manufacturers but they're both lightweight XC hardtails. If I recall correctly, the Yeti frame would be in the high 3lb range while the Trek is high 3/low 4 lb range. Personally, I'd prefer the Trek but that's mostly due to style. In terms of performance, you will notice very little if any difference between the two frames.
You'd be better off saving your money till you can go dual-suspension.
