The whole idea of picking a main is picking a character that personally appeals to your style of play. No one should be picking for you.
In general if you like a speed-based rushdown approach, go Kasumi. If you like a more defensive style of play with high rewards for good reads, you'll like Lei-Fang.
NukNuks can probably show you a lot of the basics and complexities of Lei-Fang, and probably Kasumi as well. But overall, pick the character that you enjoy playing most. If you're serious about your main, you'll be playing them a lot, so it's important to enjoy yourself thoroughly.