Since you are looking at Sendero's in either 300 win or & STW for 1000 yard shooting. I can offer this from my personal fist hand experience. I have set up 4 of the early black barreled non-flued Senderos for 1000 yard hunting and shooting. These factory rifles, when bedded well and equipped with a good scope , rail, rings, trigger job and quality hand loads will usually hold to 1 moa or close at 1000. I like a 7mm also, but would advise you look into the throat life differences in the 7 STW and a 300 Win. an also the choice of quality brass. The readily available RWS brass fr the 300 is a very good choice. If you plan to shoot a lot this may help you decide. Also if Elk are ever a posibility I like the added energy at 1000 of the heavier 210 gr 300 bullet. I have found that with the 300 win H-1000 is a awesome choice. I have never seen a temp stability issue with H-1000 and have taken elk to 800 plus yards and a coyote past 1000 with this powder and a 210 Berger. Both were one shot kills at different elevations and temps. The 300 win also has a pretty good selection of on the shelf factory ammo for choices, and can probably be found in places the STW may not, if that matters to you. Like stated by others they are both very good choices for a 1000 yard gun. One more tid bit that I was given by a very well known barrel maker, when trying to make this decision for my self on my last custon build, he stated, " Well, the 300 win holds more than it's share of records, and I haven't seen the 7mm take over and dominate yet"
Good luck, it is a hard choice and I hope I have given you some good usable info to think about.