I am a USC visiting my British boyfriend here in the UK for a few months, and we are discussing our marriage options. I'm just here as a visitor, so I have to return to the US at some point. Our plan is to live in the US. My questions are:

1. Is the visa process easier/shorter if we were married outside the US and then came in the country? Would he be allowed in the country straight away, while we are applying for the spouse visa?

2. Do I have to be in the United States to apply?

3. I'm told by a friend that it would be easier if I returned to the US and then came back over here on a fiancee visa. Does anyone know if the UK process is easier/faster? And would I actually have to return to the US to apply for that? I'm wondering because we plan to travel around Europe for a few months, and I'm wondering if I can apply for a fiancee visa when we're traveling and then return to England immediately on that.

Thanks much!

No, the visa process is not easier/shorter if you marry outside the US. He is not allowed in straight away just by virtue of being married to a USC. Also, since visas are applied for outside the US, for entering the US, you would not apply for a visa from inside the US.

The US citizen takes the first step in a family-based immigration case by filing a petition. She does not have to be inside the US to file the petition, but it must be mailed to the US and have a usable US address.

I don't see how it would be easier to move to the US by getting a UK fiancee visa, but it's generally accepted that the UK immigration process is 'easier' than the US'. Again, with a visa, you do need to apply from outside the country. The UK now requires that you have the correct visa type if you want to marry in the UK and remain.
About applying outside your own country, you'll have to look into that more. Many US Consulates will only serve you if you're a resident in their district.

Getting visa to move to the UK is faster, a lot faster.

As for times to move to the US, both routes are taking similar time frames, but there's a fair bit of info going over the visa types in the wiki...

If going down the UK fiancee (or spousal) visa route, you'd have to apply from the USA. All UK visas must be applied for from within your country of residence.

Keep in mind a UK fiancee visa is pricey ($1050 + £395 to apply for a spousal visa once married). Many couples instead marry in the USA or in a third country and then apply directly for a spousal visa to come to the UK. This also costs $1050 but saves the costs of adjusting to a spouse visa once in the UK.

Moving to the UK is A LOT faster - less than a month to process the visa compared to over 8 months to move from the UK to the US.