None of those. And don't arrive on a visitor's visa!. Before you make your journey you need to obtain a spouse visa at your nearest British Mission in the States. Armed with that spouse visa you will be able to work in the UK without needing a Work Permit.
Lots of information available by
clicking here.
Which visa type? Click on "Settlement" and then "Settlement : Spouse" on the next screen.
Edit : One further thought, given that the two of you have been married for 20 years, that is, nicely over 4 years, when you go for your visa in the States, instead of the normal two-year spouse visa you should get issued with ILE ... Indefinite Leave to Enter ... the UK. However not all British missions are totally up to speed on their ability to issue ILE and it will do no harm you mentioning ILE at your visa interview. Their ability to issue ILE has existed only since April 2003 and probably comes up only rarely in practice.
Also, just to emphasise, on no account enter the UK as a visitor or tourist and then think you can adjust your status in the UK. You can't! They would promptly tell you to go back to the States and apply for your spouse visa there.
