Another ingenious "Who's Better?" thread. Listen, there is no "Who's Better?" Seriously, it's like asking "Is apples better than oranges?"
You might get a couple of unbiased answers, but Pally players are going to say Pally, SK players are going to say SK. Pallies are...well...Pallies. Shadowknights are the anit-paladin. Pallies can heal and have beneficial spells, while Shadowknights hurt and have detrimental spells and get a pet.
The only thing that should matter is which do you have more fun playing. If I'm not mistaken, they can weild the same types of weapons and wear the same types of armor (class specific armor excluded of course).
Since SK's don't get spells till 9, I'm assuming the same holds true for Pallies. Create one of each, play them till 20, you should know by then which you like better.