For a moment there I thought the Viper was a capless! That would have been awesome. Still a nice looking pen though.

My experience of Sailor is that they make plasticy pens, but with very nice nibs. The Viper looks more substantial, but I've never used a Diplomat before.

I've never used a gold nib either.

It probably more depends on what you're looking for in a nib. Gold nibs typically have some kind of bounce to them. The Viper is a hooded steel nib so it'll probably be a much more stiff writing experience. But I've heard Diplomat nibs are pretty smooth writers. In my experience, Sailor nibs have some feedback and can "sing" on the page.

One thing worth taking into account is that the Diplomat will be a much heavier pen than the Sailor. Whether that's a good or a bad thing will come down to your preferences, but searching around reveals the body of the Diplomat weighs 21g alone, versus 10g for the Sailor.