Probably they'll be fairly equivlenet, though it really depends on the manufacuture of the chips. Higher density ussually means lower feature size IE smaller micron processes size, but does not necessarilly mean that the new processes is optomized for perfromance. Generally I'd go with the higher density chips, generally.
As an asside most of the sticks that Corasir sells are the same. For example, the PC3000, PC3200, PC3200C2 all use the same winbond CH-6 chips, and same PC board, just rated slightly differently. So which one is better I say go with the best price.
As for density, its always better to go with one stick instead of two for overclocking purposes. Adding two sticks increases the address bus loading, more address bus loading the weaker the address strobe single is that is being sent to the sticks, and the weaker the signel the more suspetible it is to noise, and the more susptible to noise the lower its overclocking head room is. Overclocking increases the frequency the ram runs at and reduces the response time and tolerance to noise.