Serco said it would improve the service with new trains, Scottish procurement and potentially more jobs.
Chief executive Rupert Soames was a vocal critic of independence as head of Aggreko, but Serco said it has no plans to invoke the seven-year break clause in the event of independence.
Scottish Transport Minister Keith Brown insisted the constitution is "not a material consideration", but confirmed the break clause had been agreed to take stock of changing economic circumstances.
Rail unions have attacked the decision to take the sleeper out of Scottish hands and give it to an English company with Spanish trains and "a truly shocking track record in the delivery of public services".
Mr Brown said ministers "had no choice", and insisted the appointment will herald "a new beginning" for the service.
Jamie Ross, business development manager at Serco, said: "We are fully committed to a 15-year contract. Our new chief executive joined a month ago and totally immersed himself in the sleeper bid.
"The break clause is something that we have not discussed, and wouldn't discuss.
"There are no plans for staff reductions. Because this is being split out from the main ScotRail franchise, if anything the total number of people employed will go up.
"There's a huge push to get local suppliers involved, most obviously in food and drink but also in getting things like Shetland blankets, so benefits flow out much wider than the network."
Mr Brown said the current sleeper is "less than ideal" and Serco offers "a dramatically improved prospect at a reduced price".
He said: "I had a conversation with Rupert yesterday. We didn't discuss Scottish independence, we just talked about the procurement process.
"(Independence) wouldn't be a material consideration for them although we have seen in the past different franchise operators essentially ditching the contracts, in the East Coast Main Line for example, although we have no intention to see this change. It's a 15-year contract and that's what we intend to see through.
"There is an option to break after seven years. Things may change over seven years. One of the problems with the UK franchise is, if you think of the East Coast Main Line fiasco, the difficulty people have in bidding for contracts well into the future judging for inflation and other economic circumstances.
"So both in this franchise and the main franchise we have built in that break period to take account of what's happening at that time, but it's not our intention to break the contract and I think Serco, who are extremely pleased to have won this, have any intention of breaking it either.
"There's nothing in the contract either on our side or on their side that says that the constitution has changed so therefore the circumstances change for the contract."
There is a "substantial financial penalty" if the contract is broken, he said.
Mick Cash, acting general secretary at the RMT union, said Serco has "a truly shocking track record in the delivery of public services", and any threat to jobs and conditions "will be met with fierce resistance including the use of industrial action".
Manuel Cortes, leader of the TSSA rail union, said: "This decision flies in the face of everything (Scottish First Minister) Alex Salmond has been saying about an independent, financially strong Scotland.
"He has taken the contract away from a Perth-based Scottish firm and handed it to an English-based firm which will be given a £60 million subsidy to build new trains in Spain. How does that create any new jobs in his dreamland of an independent Scotland?"
Mick Whelan, general secretary of train drivers' union Aslef, said: "No-one has taken on board the concerns of the staff who are currently involved, and their futures. We shall be contacting all parties on behalf of our members."
Mr Brown added: "We have no choice in (the franchise), we have to go with who wins the bid and the best bid came from Serco.
"The UK Government, Glasgow City Council and the Welsh Government deal with Serco.
"We don't like the franchise process, not least because you have the absurd anomaly whereby state-owned companies from the rest of Europe can bid but you can't have a public sector bid from the UK. It just doesn't make sense and it disadvantages UK industry.
"French, Dutch or German state-owned companies like Arriva can have a large part of the UK network but UK state-owned companies can't.
"The UK Labour Government had two chances to change that and didn't. The Scottish Government has no power to change the franchise system but hope to do it with independence."
Mr Ross added: "Serco works with all sorts of contracting models, very often partnering public sector bodies.
"The Scottish Government's aspirations to introduce new entrants, perhaps from the public sector, isn't something that we would be wary of."