Personally I dislike the look of the southdown snorkel, as it's silver section looks awful I think.
The mantec snorkel was always my favorite as it was fitted to the camel discos, and doesn't stand out too much, and although not as common, are reasonably priced.
The safari I have never really liked, because it always looking to me like the disco was wearing one of those microphones that people in call centres wear.
Having said that, I've seen a few safari ones lately and have to say in the flesh, they are a bit showey, but do look very good in the flesh.
As to performance, I'm not sure. I prefer the look of the snorkels which have a pre-cleaner on the end, instead of one of the right angled bits, but not sure which works best. I'm under the impression that the thickness of the snorkel piping is a bit of a bone of contention amongst some, as it may starve the engine, although others say that they are still bigger than the standard air intake.
To add another spanner in the works, there are a company on ebay which make safari 'style' snorkels, for about 90 quid. Apparently the instructions or crap, and the quality is a little questionable, but with proper fitting, and plenty of sealant, as long as you're not driving through the amazon rainforest on a regular basis they should hold up okay.