US citizen marrying EU National

My partner's and my marriage plans are a bit complicated and we would like advice on how to proceed in the most efficient way possible.

He is Italian and had been living in the UK for more than 5 years (that's where we first met, is a permanent resident), then moved to California around the same time I returned after my completing my studies in London. I spent almost a year in the US then returned to the UK for another 6 months to finish some work, and now am back in the US with him. We want to marry so that neither of us has any problems entering the US/ UK/ EU to join the other as he wants to start a business that requires him (well, us) to spend part of the time in the US (Silicon Valley). Where should we marry/ settle first- the US or UK? If we marry in the US how long should we wait a while before applying for a green card? Or can I be issued one at the airport when we enter the country?

Our biggest concern is having to satisfy physical residency requirements at either end. As founder and CEO of his company he would have to travel a lot anyway, but we plan to retain a house/ flat, bank account, etc. in both our names in the US. Is there a requirement that he remain in the US/ UK more than 6 months or whatever with me? Would that affect my/his residence (or marriage?) status? He would like me to be based in the UK in case of illness. At this moment he is in the US on an L-1 (executive) visa.

Our long-term plan is to live permanently in the UK.

You can apply for grrencard immediately once you marry in USA, and get it without leaving US. If you marry abroad you will have to apply for special visa and it takes time (but then you will get GC in the airport). You cannot just get marry abroad, come to US and ask for greencard at the airport.
in order not to loose GC you need to satisfy immigration officer at every entry that you did not abandon your residency in US (bank account, house, US credit cards, etc. are all good). You DONOT need to spend at least 6 months in US every year, but you better do not be absent for more then 6 months in a raw every single time (if you were abroad for less then 6 months, immigration has to prove that you abandoned residence if they want to go after you; if you were abroad for more then 6 months you have to prove that you are still residing in US).
If you want to apply for US citizenship, you can do it after 3 years on GC and here you need to be present in US For at least 18 months out of last 3 years.

Thank you.

If we marry in the States then return to the UK for a while (he has some business to take care of) will it be faster/ easier to apply for the green card theough the consulate?

Not sure. My impression is that applying for spouse green card from abroad is more painful then from within US, and there is a long waiting period, during which he may have problems entering US temporarily (as he demonstrated immigration intentions).
On the other hand, if you apply from within, they have to send him "Advanced Parol" within a month, which would allow him to travel in and out when his case is considered.
Check here
http://forums.immigration.com/
and especially here
http://www.visajourney.com/forums/