The Panasonic SD20 has higher 16x optical zoom (12x on Canon HF100). I believe the SD20 also has a touchscreen and is also 110g lighter than the HF100 at 270g (the Canon HF100 weighs 380g). From what I’ve read the Canon HF100 has better low-light performance.
Overall, I think it comes down to what price you can buy them for. The Panasonic SD20 is around £330 from QVC, which is £30 more expensive than the Canon HF100 at Currys (if you can find one). If you do find one at Currys though, it’s likely to be a display model, which may bother you. In which case, you may as well go for a sealed model from QVC.
Basically, I don’t think you’d go wrong with either model. Both have received quite good reviews.