Welcome. What a wonderful gift and gesture. I'm sure you'll get lots of folks here strongly suggesting you let your girlfriend pick the machine herself. Buying a sewing machine is much like buying a car...it's quite a personal decision really. There are many features on sewing machines that are very important to some that are totally irrelevant to others.
That said, out of the two you mention, I would go for the Viking. Singer's newer machine's don't have quite the same reputation that they did years ago. I have a slightly older version of the Sapphire and absolutely love it.
Is it possible that you could go with a higher-end used machine? Are you working with a dealer in your area? They frequently have used machines for sale as well. Even used machines - from a dealer - usually come with at least a limited warranty. Many are trade-ins and have been well serviced prior to sale.
Good luck and congrats on the new house.