For best exp gain with your setup I would go for a paladin tank, for best ability to kill named-mobs for loot I'd go with the shadowknight tank. It is quite possible to ae tank as a sk, but you are much more limited in doing so than on a paladin, and it requires a lot less fuckups from your group. Basically on the sk tanking 1-5 mobs if youre prepared for the mobs in advance isnt a problem. The problems arise when a groupmember assists on the wrong mob, and overagro's. because if you tank 5 mobs, lose agro to a caster on one mob, taking that agro back without losing agro on the mob that that guy was supposed to be assisting on and which the rest of the group are building agro on is a bitch. With a paladin tank, whoever got agro can just run to you and do stuff to lower agro while your ae spells regains agro, this without forcing you to move, slightly losing agro on all other mobs, or switch targets losing agro on the mob the rest of the group is focusing on.
Also in situations where you suddenly get 4 surprise adds, the paladin is superior in quickly controlling them (one or two adds isnt a big issue, but the more the worse the situation becomes). In groups, simply put, the paladin can hold a much safer and better control over a larger amount of mobs, spending a lot less effort. This means more exp, with less work.
For duoing, if you know the place youre at, you can easily tank 15-40 mobs on the sk and keep agro on them if you know what youre doing and play agro safely on your box. In this scenario you also have the advantage of a higher dmg shield than the paladin can get, ie higher free dmg (if the mobs arent undead in which case the pally wins again, if he has a decent enough manapool atleast).
Ohh, most of what's been said by me above and by previous posters, mainly goes for lvl 65's. Pre-60 the sk is a lot more fun and resourceful than the paladin for your groupsetup, in the long run the paladin would be the easiest choice to get good exp with.
When it comes to charisma, its definitely the way to go on the paladin. On the Sk, it isnt at all as important. However, dont make the mistake of thinking it completely unimportant. I put some points into charisma on my sk and consequently my charisma is decent for my tier. Ive heard other sk's having problems landing lifetaps, debuffs and dd's where I have no issues at all, so the charisma does matter, just not as much as it would for the paladin. You might also want to consider putting points in agility though. It's quite important for your tanking and a lot harder to come by than stamina. It will take a LONG time in gear, before youre done overcapped in agility on either paladin or sk.