Op - where is your hotel?
If money is no object, then cab it. It could get over US $100. That personally makes me itch from a "principal of it" perspective.

I take a train wherever possible, but that is my own personal preference and I enjoy the challenge. You may not feel the same, especially if arriving later in the evening.
You can take the Heathrow Express to Paddington and then cab it to your hotel. You can also check google maps to see where the closest metro stop is to your hotel from Paddington, and take the tube from there.
Google Earth and Street View can also do wonders helping to get familiar with the route before you arrive - the walk between the tube and your hotel.
If you have a crap-ton of luggage, well a cab may be best. GL!