For more low-end, you only want to add to the rear. I don't like the idea of taking teeth away from the front, because it makes the already-sharp reflection angle of the front sprocket even harder, which causes more chain wear. I recommend leaving the front alone and adding a tooth or two to the rear sprocket.
Your mileage will worsen and unless your bike has an old-skool front-hub speedo, you'll also be screwing up your speedometer readings and increasing the mileage on your odo prematurely.
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