I have seen variants of the word tschüss.
Duden lists tschüs and tschüss as correct spellings, one with a long ü and one with an short ü. Is there one variant that is preferred?
The oldest variant is the one spelt tschüs. It derives from the Latin valediction ad Deum over the French à Dieu, later adieu. In colloquial German, this was turned into tschö in some parts (because of the similarity of the French eu phoneme and German ö) but modified into tschüs in others. Going by its spelling, tschüs probably has a long ü sound (which tschö definitely has) but one cannot be fully certain.
Over time, this form merged onwards towards tschüß, making sure that the final s had to be pronounced as /s/. According to the old spelling rules (in place from 1901 to 1996), this could have been pronounced both with a long and with a short ü — the orthographic rules allowed for both.
In 1996, as a part of the reformed orthography, the distinction of whether to write ss or ß was no longer based on morpheme boundaries but only on vowel length: short vowels were to be followed by ss and long vowels by ß. Many Germans pronounce tschüss with a short vowel, hence the suggestion of writing it with a double-s. However, some people will still pronounce it with a long ü, requiring the spelling to be tschüß. If you wish, you can see two different pronunciations of the same word, giving rise to two different spellings.
Technically, the tschüs-form would still be valid for those pronouncing it with a long vowel. However, I would recommend you use either tschüß or tschüss which I perceive to be the more common spellings nowadays.
: Of course, there is terminal devoicing in German, meaning that even tschüs can only be pronounced with a terminal /s/. But that could have lost the sound /s/ in cases of derivatisation.
: If you are in Switzerland, the preferred orthography is always without ß, so tschüss in any case. However, the word tschüss is perceived as a northern word (at least in Bavaria) so I assume that using it in Switzerland will automatically put a large stamp reading German (the nationality) on your forehead. Probably similar with Austria and the Piefke stamp.