My boyfriend got me a Patagonia R1 Balaclava for my birthday and I tested it out snowshoeing in 20 degree temps over Christmas. The R1 fabric is very comfortable and did a good job at blocking the wind despite being very breathable. I wore it with my entire face showing through the opening (eyes, nose, and mouth) and found it to be perfect. My head never felt hot or cold when moving or when standing still. With this balaclava, I did not feel the need for an extra hat at all.
I haven't used other balaclavas, but my boyfriend, who works at REI and gets discounts on gear purchased from them, actually went to a Patagonia store and bought my R1 balaclava retail because he felt it was a superior product. After I let him borrow mine while building a snow shelter with his cousins, he went back and bought one for himself.
For 20 degree temps I think the R1 balaclava would suit you fine. If you are hiking in high wind or colder temps, you might want to add a beanie of some sort. I'll be testing out my gear in an upcoming trip to New Hampshire's White Mountains in February, so I'll have better insight after that.