I assume that you are looking at Currys who are selling the SD100 for £386 and the HF100 for £449. Currys also had the Panny SD9 for £299 and the well reviewed Sansumg HD camcorder (cannot remember model no.) at £299.
(Note that all the above items have prices ending in 97p so these are both discontinued by Currys and are no longer available on their web site. My local Currys ad 5 HF100s in stock last week but then one day they all dissappeared. I got mine from Bluewater and it was the last one other than the display model.)
When faced with this choice myself last week I went for the HF100.
My reasoning, according to the reviews that I have read the HF100 has better video quality, especially in lower light e.g. indoors - and I often use the camcorder for filming the kids indoors.
The SD100 has advantages (£60 cheaper, face recognition, more manual controls, viewfinder) but at the end of the day video quality was most important to me.
Have a look at the reviews e.g.
Panasonic HDC-SD100 Review and User Opinions - Panasonic - Camcorders Reviews by Digital Trends
and a personal comparison of the Panny and Canon at:
Panasonic HDC-SD100 vs. Canon HF100 | Acmelabs' Blog
Interestingly the first reviews that I have seen of the replacement models (Panasonic SD20 and Canon HF20/200) indicate that video quality was better on the old range.
Canon HF20 review (same as HF200 but with built in memory) - replacement for HF10/11/100 range
Canon Vixia HF20 Camcorder Review - Canon Flash Memory
"What did shock us, however, was how steep of a downgrade the HF20 really is. Sure, it's smaller and lighter than the HF11, but it simply doesn't have the video performance"
Panny HS20 - same as SD20 but with Hard Disk. Replaces the old Panny SD9
Panasonic HDC-HS20 Review - Camcorder Reviews - TrustedReviews
"Its performance is a little behind even the HDC-SD9 overall, and its low light abilities are noticeably inferior"
I haven't seen any reviews of the SD200 but it has the same sensors as the 300 range I think and that has been getting very good reviews.