The Vixia HG21 is almost identical to its sibling, the Vixia HF11.  It records to a hard drive, unlike HF11 that records essentially to internal memory.  There is a reasonable difference in the prices of these camcorders, mainly due to the 120 GB hard drive of HG21 and 32 GB internal memory of HF11.
The difference between the recording times is rather great, HF11 — 2 hours, 55 minutes; the HG21 — 11 hours, 5 minutes.  You can supplement both these camcorders with SDHC cards.  One of its added advantages is …