I really can't comment on the engravability of the two, however when I used to fabricate bands with 18ct nickel it was a beast to bend by hand, even at only 1.1mm thick so one can imagine that this would hold an engraved design much better than the palladium alloy, but that is not to say that the design in palladium would not be durable. To tell you the truth I am surprised that 18 nickel is still in use with the variety of alloy options available now.
The palladium engraves much nicer if you are not using assisted engraving. The nickel is very hard. If you are running some sort of air shouldn't be a problem. The nickel 18 is much whiter than the palladium white, which has some faint vestigule yellowing. (Only really visible next to white.
It will depend on which alloy was used,most nickle white is rhodium plated.The 18k palladium white can be very different materials depending on who alloyed it,Hoover & Strong alloy is very different than Hauser & Miller alloy.