I suggest you look at the legalities and complications of someone who is not a citizen of the country getting married in each one. If you search below you can find a number of threads on the requirements for getting married on Italy. I have not seen similar in terms of marrying in Greece.

In either case I would look into hiring a planner to lead you through the whole governmental process - as well as help with making location and reception arrangements.

That's not much time! I agree with the post above. Make sure you research the requirements carefully, then pick a wedding planner ASAP. The two destinations you chose are completely different, of course, so consider which one is really your style for a wedding. Have you been to Rome before? I love it, but find it very hectic and at times overwhelming. Santorini is GORGEOUS, and many other islands are on the same hydrofoil/ferry route, so it would make vacation planning fairly simple. Good luck!

Why not marry at home and take your honeymoon to Santorini and/or Rome? It will make planning so much easier and you'll get many more people who can attend. Who's going to be able (or want) to pay what it would cost to fly to either location for your wedding at this late date?

If you choose Santorini, make sure your wedding location will be private or at least protected from resort goings on. We saw a wedding at our hotel with noise from bar, very loud music, which was disturbing to those being married as well as to hotel guests watching.

Santorini is a very popular romantic island for weddings....
People from from all over the world are getting married there, and there are hotels ( i can recall Aressana) and travel agencies that can organize everything.

However mid October is almost the end of the season ( which officially ends on 31/10 ) and there is not going to be very much going on on the island....
I think you should take this into consideration....

Further to what clausar wrote: one reason October is end of season on Santorini is the unpredictable autumn weather. While you could have some warm sunny days, it could also be windy, cool, and cloudy. June, July and August in the Cyclades have almost unbroken hot sunny weather, but that begins to break down in September.

I am very much against getting wed in a place you haven't set a foot before. Moreover trusting a third part to organize what is supposed to be one of the most important days of your life. No matter if everyone involved does its best, you can not know if a place suits you before been there. And this is a huge risk to take for a wedding. If your decision is set, I agree it is advisable to look at the official practicalities before making up your mind.
I am not sure how a large city can compare with a tiny but very popular island,or what kind of wedding you have in mind, so hard for me and others to advice...
For Santorini, every now and then, a huge cat fight between wedding planners and newly wed couples seem to take place in various websites when wedding went wrong, for example check Trip Advisor Santorini forum. Sometimes there are reports and details for a successful wedding, or threads seeking assistance how to organize a wedding. Some of those threads stay put,some are deleted. Sometimes it is hard to make out which threads are genuine and which are fake, but I guess it is somewhere to start. Sorry but I haven't see a related thread here, so check TA Santorini or TA Wedding Destinations forum for some pointers..

There is only one wedding hotel in Santorini, it's called Astra Suites. There are several celebrities got married there.

It has one of the best views in the world and the weddings are taking place during the sunset so the photo to be extraordinary.

Google it
thx
Giakomo

Giakomo,

this is not true at all....
There are no wedding hotels.. Santorini has many excellent hotels that would be ideal for a wedding/honeymoon....
There are local travel agencies that can take care of the formalities and organize everything.

But i guess that Giakomo is working at the hotel, so he has no other choice than to recommend it unconditionally....

Do you expect friends to come to a wedding in a foreign country less than three months from now? Or are you trying to get away from your family and friends to keep the size of the wedding down?

In a lot of companies it is too late to get a serious amount of vacation less than three months in advance, to say nothing of the costs of travel and lodging. We are going to a wedding in overseas this fall. The bride and groom are nationals in that, so it is not a destination wedding. Four of us will spend =/- $4800 on airfare and another $1600 on an apartment, much cheaper than a hotel, and that doesn't include any of the other costs like cabs, food, etc. Can your friends afford this? Unless your friends don't work and you are flying them over in your Gulfstream V, of course.

Do you expect them to spend the $$$ to come to the wedding AND send you a present, or have you told them that their being there is present enough?

Seriously. Get married at home, go to Rome (it's probably too near the end of the season for Santorini) and have big party when you get home.