I have five little ones now. You love them each just as much, only differently. The new ones you love just as much, but you don't know them as well. So, at first, I could see how you might think you love the others more...but in time you'll grow to fall in love with any and all you have.
For me personally, I fell in love quickly with each that came along...just that there was a phase of getting to know them. My newest baby is only two weeks old, and we are all smitten by her.
My first was my most difficult. I've always been glad because it's made each additional one seem easier. I'm not sure how it would work to have a harder second child. I think since you're aware of favorites, you'll be less likely to have a favorite. I don't have a favorite or a least favorite with my five. It's totally possible to love them all just as much for different reasons.
When it comes to boys and girls, there is a special bond between mother and son. I grew up with only having sisters. I didn't know what it was like to have a boy. No idea. My 2nd was my son, and we have a very special bond. It's like the father/daughter bond, only mother/son. I didn't know that type of bond existed! I love my four girls just as much as my boy, only different. I love them each different and just as much.
I really think you'll find you'll be okay:-) Even if he's a more difficult baby, you'll feel a tremendous amount of love. Just don't freak out if you don't feel as deep of a love immediately...you've had two years to get to know your daughter. Your new baby will be new, and I know it can take a little bit of time for some to develop that deep love. It doesn't mean you are favoring one over the other or anything like that.
It's normal to have worries and stresses when ti comes to new babies! You always wonder how it'll work out, but in the end, it always does:-) Hang in there...I'm sure you'll be blown away with how much you love your boy!