IMO, the biggest advantage to buying from the developer (or buying a mandatory resort) is the ability to use SVN, which is Starwood's internal exchange system, so that you can most easily exchange into other Starwood resorts. Not being able to use SVN is a huge disadvantage to buying a voluntary resort on the resale market. If you are interesting in exchanging into the top-tier Starwood resorts -- Harborside Resort at Atlantis (HRA), Westin St. John (WSJ), or the ski resorts during ski season -- possibly your only bet is to use SVN, especially during high season. This is why the mandatory resorts hold up their resale value better than voluntary resorts.
However, as DeniseM stated, Starwood doesn't have timeshares in Europe/Asia. So SVN is useless in this regard. In order to exchange internationally, you have to deposit your week with an external exchange system, the two most popular being Interval International (II) and RCI. And then you have to hope/wait for availability to open up at the place you want to exchange to.
I have no experience trying to exchange internationally. However, if you can give us a few names of places that you'd like to exchange to overseas, I'm sure Tuggers would be happy to do a test and see if our weeks "see" that resort in II. That will give you an early indicator as to whether this will be an easy exchange or a difficult exchange.
In II or RCI, ALL owners are treated the same, no matter if you paid $1 on the resale market or $50,000 from the developer. What does matter are: 1) Planning ahead and depositing your week early; 2) Owning a week with a high trading power; and 3) Owning with a company that gives you priority over other II members.
#1 is easy for people who plan ahead; not so easy for people who don't. You can deposit up to one year in advance of your use year. The earlier you deposit, the better.
#2 is key. High trading power is essential if you want to snare the best exchanges.
#3 This is a perk of owning with one company over another. Let's say Starwood deposits 20 weeks at Westin Ka'anapali (WKORV) into II. In theory, any II member who owned a timeshare above a certain trading power would be able to see it. However, both Starwood and Marriott have a built-in advantage for their owners only in II. IIRC, Starwood's is 3 days and Marriott's is 20 days. Only when the priority has expired will other members even be able to see those weeks. So most of those 20 WKORV weeks are going to be grabbed up by Starwood owners before they ever make it to the general II population. Marriott owners are at a major disadvantage for getting Starwood weeks... and vice-versa.
Since Starwood doesn't have any international timeshares, you will get no advantage for international trades. However, Marriott does have a few international resorts. So if a resort such as Marriott Marbella (in Spain) or Marriott Phuket (Thailand) intrigues you, then you should really look more closely at the Marriott system.