Hello, another only here. I was very close to my parents in a lot of ways and I think there are definite advantages to being an only child, but I definitely want my dd to have a sibling. There are 2 main reasons, which are more due to my "situation" as a child than being an only, I think, so here goes...
My parents went out every Saturday night and left me with some boring teenage babysitter, or alone when I was old enough. On Sundays often we wouldn't do anything most of the time as they were hung over. Often on holidays or daytrips we would go to the pub during the day, and I remember so many boring times sitting quietly with a colouring book. Yes I was good at playing alone and now am very strong, independent and resourceful as a result of all the time alone, but I was so lonely and bored.
I also soo agree with what TsarChasm said about 'old heads on young shoulders' and being introspective. This has made me independent and academically a high achiever, but I didn't feel happy as a child with my "old head" and would have loved more silliness, noise, chaos, and not feeling soo guilty when I'd done something naughty, because if I'd had a sibling I would have seen them do naughty things too.
So I think it's VERY important to ensure that an only gets to do lots of child-centred activities like going to the park loads (that the balance is right), that they are surrounded by other children as much as possible and that they get lots of opportunities to be silly, chaotic, noisy, and dare I say it, naughty.
Sorry if this is direct, but you say that your careers take up a lot of your time and are very demanding - doesn't that mean it is even more important that you have another dc so your ds won't be terribly lonely? An only child will crave your constant love and attention and will feel very lonely if he doesn't get it... BTW I understand what it's like to live abroad and have no family around as I'm in the same situation - it's hard!