I'm assuming BOTH! is not a good answer for you.
Anyway, what would you like it to do?
Do you want to take it places? The FW is much more portable than the 66. That can be a drawback too if you want to quilt with it--it really is small! (My avatar shows my FW next to a 5 gallon bucket of strings. SMALL!) I get a kick out of my FW each time I get to use it. I can drag it to my mothers, set it up on a little old-fashioned typing table and sew away. However, I wouldn't attempt to FMQ large amounts with it. The closest I've come to quilting is making some mini potholders for my niece's play kitchen.
The 66 is much more sturdy and pieces well. It will have more room to manage larger pieces. It's not one I want to lug around. I also have fun with my treadled Red Eye. I haven't attempted FMQ with it. Some say the way the bobbin is situated (horizontal) makes it more difficult, but not impossible. I have tried FMQ with a 201 (same bobbin setup), and I seem to get better results from my 15 or 301 (vertical bobbins). Straight line quilting is just fine for me with the horizontal 66 bobbin setup.
It really depends on what you need...or what you fall in love with! FWs are really cute, and they can steal your heart quick.
Remember, there are lots of vintage sewing machines out there, and they all seem to show up once you start looking. If you have the patience and are limited in resources or space and can only get 1, you might want to wait for a machine that would suit your needs better. (I personally would choose my 15-91 if I could have only one machine. A shortbed 301 with a cabinet would come in 2nd. Others would have different opinions).