It really depends on how you throw the ball.
For myself, I have a softer hand at the bottom of the swing, so Rg and differential really plays into what a ball will do for me. So for me, right now, by best ball down is a Light's Out with surface on it. I can see where the Wipe Out will be a good fit under the Zero Gravity, for me. Again, it all depends on how you throw it. I can get "weaker balls" to hook more than what speed dominate players can. For me, low RG balls read the lane much quicker so that puts the Tour balls in an odd place, not really a ball down, other than the cover being a little cleaner through the first few feet of the lane.
It really depends on how you throw it, and if your style needs to rely more on cover strength or core dynamics as being the deciding factor as what ball to go to. I know that the industry says that cover makes up the most important factor in ball motion, BUT, often times people really over look core dynamics as well. Cover and cover preparation dictates when and how soon or late a ball will slow down, but cores define the shape of motion as that ball is slowing down. This is why the IQ Tour solid is different from the soon to be released Wipe Out, and is different from a Victory Road solid, etc.. any ball that has R2S solid on it. Same cover, but different cores dictate different shapes on the lane. The R2S certainly seems to be a step down from the ERG solid cover, in my opinion.