I did this Australia holiday with HK add on earlier this year and I can totally recommend both. When on holiday I rather like 'my hand held' without having to do any thinking for myself and was also nervous about HK without a tour guide.
It turned out brilliantly! We were escorted from the airport to the hotel in a large chauffer-driven Mercedes and were picked up at the end. There was a group of us who met up for outings, dinners etc. Apart from that, HK is a fascinating (cheap) place and you will be missing a great experience. Taxis are cheap and very plentiful but get a card from the hotel in Chinese as taxi drivers tend not to speak English.
If you do decide to book your own excursions, the hotel will do this for you and you will be picked up and dropped back at your hotel.
Having travelled all that way, the HK add on is not to be missed!
Hope this helps.