We sell both and have tested them. They do offer a decent compromise for some one that doesn't want to go all out on the installed car kit.
The SP 100 is louder and comes equiped with a vehicle charger, home charger and various mounting clips. The HF 800 comes with a home charger.
For the full install I like the Parrot systems. They seem to be the most compatible with a variety of bluetooth phones and I have tested it with my 7100t and it worked well.
In terms of headets my favorite is the M3500 Plantronics... it is not the most comfortable but fidelity is better than any of the others.
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