If your arrival is 8pm and you want to go to the Westin Kyoto that evening, I would say that the best option is the bus that goes all the way to the Westin. If you miss that, then the next best option may be the later bus to Kyoto Station (and from there take a cab to the Westin).

If you have an aversion to bus travel, you could do this:

Monorail from Osaka Airport (Itami) to Hotarugaike
Hankyu Takarazuka Line from Hotarugaike to Juso
Hankyu Kyoto Line from Juso to Kawaramachi
Taxi from Kawaramachi to the Westin Kyoto

Takes about 80 mins.... but too complicated for your first night in Japan.
(You could take a taxi from the airport to Juso if you wanted to make it simpler. That would be roughly the same taxi ride as going to the Westin).

Perhaps it's worth considering a night in Osaka. Grab a cab to the Westin. Go to the top of the Umeda Sky Building to get a night view of the city. Enjoy a cocktail in the hotel bar. And then get some sleep and wake up good and early for some tourism in Osaka before taking the train to Kyoto. Now that I think about it, that might be the most enjoyable way to start your trip.