If there are two of you plus luggage I would taxi all the way to the airport Saturday morning. Traffic should not be an issue base on historical data.
SPG there is the W at West Kowloon and Sheraton at Tsim Sha Tsui.
Marriot there is JW at Admiralty, RC at West Kowloon and Renaissance Hong Kong Harbourview.
+1 about Causeway Bay and Wan Chai being more HK flavour. Tsim Sha Tsui is too touristy and West Kowloon too sterile and off the beaten path.
The driver needs his permit and the vehicle needs its permit to go from SZ to HK.If not have them take you to the train at Lo Wu. There are plenty of taxis at the Hung Hom (Hong Kong terminus) station. There are different taxi queue depending on whether you are heading to HK island side or staying on Kowloon side.