Hands down and without a doubt CO2 if you can do it safely. Just literally killed about 95% of my spider mite problem this afternoon on soon to be week 6 plants. I kept reading online about it and how it sucked or worked or sucked but kinda worked, etc. until I came across a comment that really put things into perspective. Everyone recommends 15,000ppm to kill the mites; that's 1.5%. Humans start feeling some symptoms from 5-8% and can work in environments with up to 30% as per federal regulations. So obviously you won't kill the mites with 15,000ppm.
I ended up dumping a 20lb tank of CO2 in a 350ft3 tent (so roughly 500,000ppm). I don't recommend anyone do this but in my case my CO2 regulator was too slow to ever come close to reaching those levels so I secured the CO2 tank near the fan, sealed everything up, twisted that cylinder, finished closing up the tent and ran out of the structure. Notice I'm saying "structure", as in a separate building, I would never advise anyone to do this inside their own homes or even garages. 175ft3 of CO2 is no joke. DO NOT PLAY AROUND WITH IT. That being said, it took me nearly two hours to find a live mite. Seriously, I couldn't believe it. 1 application and they were at least 95%+ gone. I still don't know how it works on the eggs, etc. but will find out.
As for the plants? I ran them in that CO2, lights on, for 2 hours and they came out looking so much better. I was going nuts over how to deal with the mites and really didn't want to sacrifice any plants so close to harvest. This was my last shotgun attempt and low and behold, a natural pesticide that won't damage your final product in the slightest. Also, follow all the sanitary steps you can think about before entering your grow area. Most importantly, clean everything that comes from the grow shop, especially equipment that has been in other grows such as CO2 tanks.
Good luck.
Edit: Although this method is, in my opinion, THE way to go during the flowering period, I think it's overkill for younger plants in the sense of its cost and risks. Also, no idea how younger plants would hold up to those levels of CO2 but mature plants can take it no sweat.