The existence of a GT6 on PS4 seems conditional on not getting everything PD want in GT6 working on PS3. In that sense, if a PS4 version is announced, the PS3 version cannot be considered "complete". That may mean GT6 on PS4 is worthwhile, depending on the time scale between GT6 PS3 (and the launch of PS4), the PS4 version of GT6 and then GT7.
It might also mean, for those without a PS3, that GT6 isn't worthwhile on PS3. I can't see an argument, except to save money / trade-in for a PS4 (or XBone / whatever), against getting GT6 on PS3 either way for those that already have a PS3, unless the game disappoints massively somehow.
So, practically, the answer is probably "both" for those with a PS3 and looking to get a PS4 at some point (unless there's only one version, i.e. PS3 only), as presumably people will be wanting GT7 at some point as well, and a GT6 on PS4 (bearing in mind what I said in the first paragraph) should tide things over nicely until GT7 actually arrives.
I personally think there should be some kind of incentive for PS3 GT6 owners to get the PS4 version, if it materialises, perhaps discounted in some way, or through PS-Gaikai (I can see Sony using it to lure PS3 owners to PS4, by offering demos of PS4 games on PS3 via remote streaming) etc.