The Tata Sierra is a great product. It is one of the best in the segment for space and comfort, it is loaded with features, and it will be a solid upgrade from your current Maruti Suzuki Swift. However, when it comes to mileage, the Sierra is not the best across its engine and gearbox options.
So, considering your priorities of fuel efficiency and low service costs, we would recommend the Maruti Suzuki Victoris Strong Hybrid. It delivers excellent mileage and you can expect over 20kmpl in both city and highway driving. Plus, since it is a Maruti Suzuki, after sales support and running costs will be very manageable, something you would already be familiar with as a Swift owner. It is also a 5 star BNCAP rated car, which adds extra peace of mind.
If you are willing to wait and want an option beyond Maruti Suzuki, the new Renault Duster is expected to get a strong hybrid around Diwali this year, and it could also fit your requirements well.