The difference in performance & fuel economy between 35s and 37s is going to be minimal. So it's mainly a personal preference. Visually, the JKU looks more at home on 37s IMHO. Just make sure you gear accordingly. If ever plan to run 37s, then make sure to gear deeper. You didn't mention what trans, but if auto, I'd suggest 4.88's for running 37s. If 35s and no bigger, than stick to 4.56 gearing.
And you don't have to go with a 3.5"+ lift to fit 37s. 2.5" lift and flat flares will get you there just fine. I've run a 2.5" Rock Krawler stock mod lift for almost 3 years on 37s, both as a daily driver and now as a driveway toy and it works just fine. Up until a week ago, I've had cut stock flares on my Jeep. Currently installing a set of Poison Spyder Crusher steel flares. Rears are on; fronts are off and primered, ready for bed liner. Cut the front pinch seam on the rear wheel well, but it's already cut from the factory on all 2013+ models. I still need to cut the rear-most pinch seam on the rear wheel wells, though, because my tires are still getting into them when I have weight in the rear of the Jeep (i.e. passengers).
Long story short....35's or 37's is a personal preference, as performance differences between the two will be minimal on the highway. Additional clearance on the trail does make a difference, though. So it's really up to you.....
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