advice on Snaffle bit style and thickness for horse
 

I have looked at the english bit guide and it was helpful, but i still have questions. I have been told my hands are relatively quiet and gentle, I try to be very conscientious of my hands.
I am looking for a bit to start my 2yo filly on, she is in a 1 joint pretty narrow (thickness) and light snaffle now and i have driven her in it a few times and had it on her a dozen or so times. I wanted to make sure it doesnt "crackerjack" her so i want to switch her to a 2 joint snaffle. Still trying to decide on the cheek piece to use a eggbutt or a d ring. Also how do I know what thickness to look at? i have heard thicker is gentler but not when you take into account the heavier weight.
I also have another horse that I ride (not mine) and she is in a 2 joint snaffle (middle is a roller) with shanks. She neck reins well. She does well in this bit but I would like to try a gentler bit on her and the owner gave me permission to try any bits I like on her. She is 15 yo and has done a lot of different riding in her background with other owners. The only fault I have found is she is barn sour and pulls on the bit more on the way home, but nothing i havent been able to handle. I therefore have the same question, what thickness of the bit and cheek style should I be considering for her since she is older and more trained horse and so far has been very good about trying other bits.
I dont think they have the same sized mouth so I am probably looking at buying two different bits.
Lastly, I used to ride english 15 years ago (rode my horse in a Dr. Bristol). Got back into horses recently, so I am new to western riding but have been riding western for a year but I want to also ride english on occasion. I want to ride in the same bit (snaffle) for both riding styles, so does that mean the contact on the reins changes?
Thanks for advice in advance.