To the brunswick gurus and people with experience using the same ball, how does the karma urethane compare with the slingshot? Is it slightly weaker, or much weaker?
If there is some oil on the lanes, will the urethane burn earlier as well? 

In my experiences, the Karma Urethane will have a smoother reaction because of the solid coverstock.  The Slingshot will turn harder off of the friction with its polished pearl cover. The Karma Urethane and slingshot can be used on shorter patterns or lighter volume oil to control the reaction on the lane. Overall, the Karma Urethane would be considered slightly weaker than the slingshot with a similar drilling in each. I am a high rev player and both of these balls allow me to play farther right on the lanes. If you are looking for a smooth reaction, either one would be good.

Our local shop has them priced within $10 of each other.  As far as being adjustable I'd say the Slingshot.
 
Urethane balls are somewhat limited because their slower response to friction.  If you want to play straighter lines on drier lanes with something ultra predictable, then the Karma Urethane may be your ticket.  The Slingshots are also great at playing on the dry, but they will give you a little more "kick" on the back end due to a slightly faster response to friction. 

Urethane balls fill out an arsenal for those who see a wide variety of conditions, but if you bowl mostly fresh conditions the Slingshot might get more use, in my estimation.  Both are good when used at the right times, just a little different because the Urethane reads earlier and smoooooooth, while any pearl reactive will read later and somewhat more angular late.  I hope this helps.

Since I am often faced with many different lane conditions, I think Im goin with the urethane.  Im rev dominant and pretty often I am faced with EXTREMELY short oil hence something that response much slower should suit me better... If its still strong I can move up in grit or slap some slip agent to kill it even more.

*Searched but there is no Karma urethane available in Japan it seems, guess the distributor doesnt want to carry this*