I agree with most comments in this thread so far.
It really depends on what you want to do with your tank and if you plan on boxing what class you choose to pair with. Here is my quick take having mained both (much more time spent on my Paladin).
XP -- Paladins have the advantage in the early-mid tiers here as they are much more user friendly and have the ability to generate aggro on more than 1 mob at once for a moderate mana cost. At the very high end (tiers 9-12) Shadowknights start to pull away with their superior aggro generation which translates to superior mana efficiency. Just ask any healer that I have ever played with they will choose to box a SK over a Paladin for XP.
Raids -- Shadowknights have the edge here. They are able to mitigate raw damage better than a Paladin can, and a Paladin really won't start to shine in raids until they start getting tomes/healing increment. HOWEVER, once a Paladin gets healing increment 6+, tomes of power, and some mana.. they can be really really amazing in a support role. But don't expect to tank many bosses in the high end unless you are very overgeared for it. Shadowknights in the high end excel more as tanks with far superior aggro generation and mitigation, and when not tanking they can put out some absolutely disgusting DPS with certain quested spells/tomes/a good 2hander.
6 Man -- I didn't really do much 6 man content in the early game outside of cmal 3 and tmaps. Both classes fare well in these situations but Paladins definitely are the better tank for tmaps unless you have CC. For the high end, Paladins are pretty amazing with their ability to pad heal low group members to prevent premature group heals, having an okay group HoT, and having a slew of awesome abilities in Lay on Hands, Hands of Piety, and Protection of the Lady (rendered useless in the very high end of 6 mans though).
Duo/Farm/Quest -- Shadowknights have the edge here at any point in the game. With their far superior DPS and ability to feign death/mez stance/leech stance etc. they really shine in these areas. (Unless you are farming/questing/duoing anywhere with undead).
http://forum.shardsofdalaya.com/showthread.php?t=21000 Pretty much sums up the differences between paladins and shadowknights.
However, whatever you do, do not use the list Otcho linked as a reference as it is outdated. Since that list has been made there have been changes made to Shadowknights, Druids, Enchanters and Bards to name a few. MAD PROPS ON THE LIST THOUGH CINN DAP DAP DADAP