What offset?

[MoisesZ71]

i will be putting some torsion keys and 2" spacers to be able to fit some bigger tires and wheels on my 2002 tahoe z71. im wanting the trail grapplers 295/70/18. i know the keys and spacers give just a good amount of clearance, or so ive heard, but what offset should i go with and what would be the difference? i want to try to get away with no rubbing and if any very little trimming. that being said, should i go with the +18mm or the -12 mm offsets? what would be the difference on them?

 

[blueflamed03]

+15-18mm