I prefer to straddle, for two reasons. Reason number one is that in that position, I have a lot more leverage and control over my own movement, while I tend to topple backwards in the squatting position. Also, squatting for too long always makes my legs go numb, and at some point I can't move; so altogether, straddling allows me to move better, quicker and with more strength.
The other reason is that in a straddling position, I have better access to the male upper body to do some manual teasing and stimulating, in a squatting position my knees are in my way.