The cost of doing both vs one at a time is not going to save you any money. the other wheel bearing may last another five years!!
I would do only the one going bad now.
IF you need brakes and all that, then doing them together might save money.
If you do not need a full brake replacement, then I would only do the one.
Why fix what is not broken?
And if noise does not bother you, you can go YEARS without fixing a bearing on a wheel.
(I drove my Contour SVt for over a year with bad fronts. and i sold it and the local person who bought it a year ago still drives it with both front bearings rumbling.I didn't care. they don't care.
Not a problem.)
A wheel bearing is way different that an engine bearing.
It can make a LOT of noise, and long as it turns and does not get super hot while driving.. it is good enough!!!
save your $$$
Added: and it is not going to cut into your mpgs any either...