Your final choice depends on a lot of different considerations. If you are looking just at ballistics and performance of the round, I would look to the 7 WSM over the 270 WSM because of the greater selection of bullets. However, if popularity of a round (availability of ammunition) is important to you, the 270 WSN may be ahead of the 7mm WSM.
Over the past several years many new calibers have been developed, but it is highly unlikely that all will survive. Just look at the WSSM family. At present, I am not aware of any manufacturers currently producing rifles in any of the WSSM calibers. They appear to be a dying breed, so future availability of brass & ammo is in question. Winchester will not even work on/replace WSSM barrels on any of their old Winchester rifles.
In the WSM family there are four calibers: 270, 7mm, 300 & 325. Like the WSSM family, many rifle manufactures jumped on the band wagon and produced rifles for all calibers. However, only Browning continue to make rifles in all four calibers. Winchester has already dropped the 7mm WSM from its offerings. It seems the balance of rifle makers that produce rifles in WSM chamberings (including Sako/Tikka) have dropped the 7mm and 325 lines and now just seem to be focusing on the 270 WSM and the 300 WSM. It would appear as if 7mm WSM brass/ammo may be difficult to find. Reloading would give the 7mm WSM extended life - assuming that 300 WSM brass remains widely available.
