The price on 28s has been coming up in my area. Hard to find a
27 for less than $1K, pushing up prices on 28. The 4" 28 seems
to be more desirable on the market and is bringing a little more
than 6" all things being the same. So best deals are on 6" 28s.
The 28 vs 27 has already been explained, it's apples and apples
in reguard to practical use.