I almost wish you hadn't asked this question. I just drilled up both, with the Blue/Black Polar Ice intended as a spare ball and the SN (75x 4.75 x45) intended as a dry lanes strike ball.
(Keep in mind that I am just returning to bowling after a 3 month "injured reserve" for a sprained slide ankle; so my ball speed and rev rate aren't up to normal yet.)

I've used the SN twice in practice session and its reaction so far is almost hyper sensitive to any carrydown or spottiness. If it sniffs oil in the breakpoint area, it doesn't seem to make the turn. So much so that the Polar Ice seems more consistent in the small hook I like for most spares. Since I throw it all over the lane, to make spares: 2/4/5, 4 pins, 10 pins, 3/6/10s and so on, it hits almost every board in the house at one time or another. So it's bound to hit oily spots here and there. (Remember that the SN and the Polar Ice use 2 different urethane formulations!)

This is only an initial report and it could be the summer oil or carrydown from open bowling AND I'm just not up to par yet. But I don't like the SN's reaction so far. I just joined a summer league to bowl on regular oil; they oil just before league starts. There was plenty of oil, too much for a urethane for my current release. I used a 4000 grit Shout and intend to bring a 4000 grit Karma solid and an IQ Tour Pearl next week to this league. 

With your rev rate, the SN should be a much more appropriate ball. My normal rate is in the 300 - 350 range; my current rate is probably 250 - 275, roughly. These lanes are usually on the drier side but the SN's reaction was so bad, I was actually considering adding some surface. It's too early to do that, though.

I retired plastic and it is not my spare ball all the time. And I use it some on hooking shots for strikes.

I have a hard time imagining the need for 2 urethane balls under almost any circumstance.  I have owned the original Natural for 2 years.  It has totally replaced my plastic spare ball.  I also use on some house shots where my low rev rate just labors too much to carry when I have to move in and throw out around the puddle in the middle.  I have also used it on some low volume short patterns where I could play outside 5 and square up.  The last thing I want to see this ball do is hook enough that I have to go away from the pocket. 

Also I have baked the oil out of the Natural 4 times. 

I see your point, but I would still think a low end, low flare resin ball might be preferable to a second urethane, even for higher rev players.  The big difference between urethane and resin is not how they react off the dry, but how little carry down it takes to impact a urethane ball.     

I just recently got a red urethane groove and love it. It has a pancake weight block and works great for spares and responds well for a strike ball on the right condition. Even with a strong drilling the ball doesn't over react. The lack of core wont allow it.