Lots of folks, including me, have been to both.
You can have a fun holiday at either. Bear in mind, however, that Hong Kong is a smallish place, although you can also spend a night or two in Macao. Other than that, as a US citizen, you need a China visa to go anywhere (with very limited exceptions, but few westerners go to places like Hainan island from HK). Chinese visas cost $150+ and take effort to get.
I personally am not sure there is 8 days in Hong Kong and Macao, but if you have 8 days in Japan, there are lots of places to go to other than Tokyo.
If staying on Hilton points is important, you could, for example, fly into KIX and stay at the Hilton in Osaka and commute (it's inexpensive and about 30 minutes) by train to Kyoto and Nara. Or you could go for the Hilton resort in Odawara, south of Tokyo.