I'm a bit confused. The devices you mention are dual-flash type devices: one built-in and a slot for adding another media card (SD). They are not hard-drive based as your question states.
If you think they are hard drive hybrids you might wish to go back and re-look at the specifications of these units.
However, all that aside, my personal opinion has been to avoid units with built-in recording media. I only recommend this route because if anything goes awry with the built-in media it means a trip to the manufacturer for repair, which could be expensive outside of warranty. If you stick with a device that only accepts your media card(s) and your card goes bad then your only out the cost of a new card.
I suppose the up-side to having built in memory would be that you don't have to worry about remembering to insert your memory card and that way you don't get caught without recording memory in a pinch.
I'm looking for an HD camcorder and am trying to decide whether it's worth the extra $30-60 (depending on seller) to go from the flash drive only Canon Vixia HF M300 to the Canon Vixia HF M30 with 8 gb or go further spending an additional $130-170 for the M31 with 32 gb. This will be my first camcorder so I have no clue as to the value of the hard drive issue. Thoughts?