Hello, I'm new here and I know this post is a lil old but hey it's got something I know, Tonfa's. Now luckily my bestfriend uses Sai's so I got an equal vantage point here. Now in the right hands it is highly unlikely for anything less then a sword to get past the tonfa's block, it's range can vary depending on your preffered size, I myself use a 24", therefore you can attack pretty easily, also it's a little easier to use a one arm block one arm attack method with the tonfa against a bo then the sai. I'm sure you all know this, oh one more point, the tonfa would give even a novice a semi-fair chance against a bo. Now the sai on the other hand if used with speed and accuracy can disarm the bo and dispatch the opponent rather quickly, down side is predict the wrong move such as going for a high X-block and catch a wooden shaft the groin, or legs, go for a low X - Block and it's goodnight everybody lights out! So I'd say for novice's and above tonfa, for advanced martial artist and above sai. The choice is really yours, it also depends on your personal abilities, I tend to go with tonfa because I have more strenght then lightning fast speed, my friend is insanely fast with less strenght, sai works great for him. So there you have it, my insight on it all, oh yea btw I pick tonfa.
