The thing we're trying to figure out is if we're better off trying to get me admitted to stay long term as a partner or a spouse. We've been together for eight years, and have records showing bills and a joint savings account. We've also owned a house together for the last four. Which do you think is better, to try to get a visa based on a longer time we can prove being together although not married, or if we are actually legally married? While they may not allow me as a partner, I'm concerned that if we do get married here, then try to move, they won't allow me because they'll only see that we got married right before the move, and won't take into account the time we've been together before that.
If you have been in a relationship and living together in the US for more than 4 years (and you can prove this), you can qualify for UK permanent residence (ILE) immediately.
In order to be issued ILE you must have passed the Life in the UK (KOL) test - however, the test can only be taken in the UK, so if you are able to visit the UK before you apply for the visa and take the test while you are there, you can apply for ILE (unmarried partner visa application, but requesting ILE) and will have permanent residence right away.
If you are not able to take the test in the UK before applying, you can apply for an unmarried partner visa with KOL Req'd stamp - this allows you to apply for ILR (permanent residence, applied for while living in the UK) once you have moved, as soon as you have taken the test. You will need to request this on your application and provide proof of living together for 4 years.
If you had not been living together for 4 years, you would not be able to qualify for ILE/ILR until you had been living in the UK for 2 years on an unmarried partner visa.
From the visa application guidance(
http://www.ukvisas.gov.uk/en/howtoapply/infs/settlementguide#87144682):
If you and your unmarried or same-sex partner have been living together outside the UK for four years or more and you meet all the necessary requirements to stay permanently in the UK, you may be granted indefinite leave to enter. These requirements include showing that you have the necessary level of knowledge of the English language and life in the UK (see the relevant section of this guide for more information).