The 20s are more pricey to replace too. That's why I went with the 18s on my RAM.
Up to you though.
As an Example:
BFG All terrain 275/70/18s 3640# max load - $208 each on tirerack.com
BFG All terrain 275/65/20s 3750# max load - $298 each on tirerack.com
That's almost $400 more for a set of 4.
The 65 and 70 refer to the ratio of the sidewall height to tread width.
The tread width is 275mm for both, so the 20" tires have a sidewall 65% as high as the tread is wide, so 275 x 0.65 = 178.75mm tall = 7.03 inches.
275 x 0.7 = 192.5mm tall = 7.57 inches.
Overall tire diameter/height:
275/70/18 = 33.2"
275/65/20 = 34.1"