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With the 20s you will get quicker acceleration but lower top speed. Plus you will get a little more tourque with the 20s. With the 22s you will lose acceleration but gain some top speed. A Bonus is you will get the 2 more inches of ground clearance.
Ground clearance is the big problem. With a 20" tire vs. 22" you loose 1" of ground clearance. To me that means alot. You can always adjust the gearing to the tire.