Sir Jeremy Greenstock, who served as the UK's former ambassador to the UN and Tony Blair's special envoy to Iraq, said the independence referendum offers Scotland the choice of being a small state with a "big Scottish label", or a less distinct region with the weight of the UK's population behind it.
"I see the product as being roughly the same," he told a meeting of the United Nations Association of Scotland at Holyrood today.
Even the US could find itself marginalised in the world if it fields poor diplomats, he said, but smaller countries with "a very powerful individual" at the helm can be more influential.
But self-determination has its limits and can leave small nations exposed to the "larger storms of the global level" if its leadership is weak, he said.
Sir Jeremy, who is half-Scottish, said he would be personally disappointed if Scotland left the UK, having viewed with dismay the growing influence of "identity politics" over global unity around the world.
But he said he could live with either outcome and rejected the notion that one is inherently "better" than the other.
"I wouldn't choose the word 'better', it's a choice of what you want," he said.
"Don't expect a change except in your feeling of Scottishness. The smaller populations represented by member states in the UN have a distinct voice and are recognised with the label of their national name on it.
"That could be more influential if they play their cards cleverly, have a particular national attribute like a lot of oil or some other natural resource, or be the gateway to something important for the larger states.
"Or they could have a very powerful individual as their representative at the UN.
"The US could have an individual who fails to project themselves and lessens American power.
"But you would need to take account of the fact that losing the voice of the UK in your interests would be accountable. You could count the difference.
"Having your label as Scotland would appear bigger to you than it would be to the audience of the UN General Assembly.
"Those are considerations which you would have to look at subjectively, and I am not going to say as an Englishman which is better."
Sir Jeremy said Scotland would secure independent membership of the UN "quite quickly with the willing co-operation of the rump UK", but it is unlikely to get a seat on the UN Security Council.
He said: "I personally can live with either answer from the Scottish people.
"I think I would regret not having Scotland as part of me geopolitically, since I am half-Scottish, and I would regret it from a diplomatic point of view because the Scots nation has contributed an enormous amount to British diplomacy and the handling of foreign affairs."
He continued: "World politics has become much more about identity politics. In Scotland, it comes from a stronger feeling of Scottish identity, and wanting to project that into the world."
But he added: "Is there an understanding of the limitations of the concept of self-determination?
"At what point should self-determination stop and people realise that they have to be part of the global framework to guard their freedoms, rights and preferred objectives?
"Unless your representatives are put under pressure to get the framework right, your individual mosaic of freedom and local identity will not be protected from the larger storms of the global level.
"At the moment the trend, to my disappointment, is away from democracy at a global level."