Hiya, Mrscee,
I would not really compare these two, or if so, I would need to know what to compare I mean, these are two very different pushchairs - the City Sights being reversible, it offers less seat space (however, the option to face the parent is suitable for those with a smaller child), higher weight (based on the travel system-ready frame), and smaller, more urban front wheels.
I personally would go for the City Mini GT2 if I wanted a lighter, long-lasting pushchair that folds compactly (the City Sights is bulkier and heavier as it is a two-piece pram) as the seat is much larger, and as it only faces forward I would want it for a toddler more than a baby. The City Sights I would get as a full-featured from-birth option (kind of like a more compact Uppababy Cruz), most probably with a carrycot, as that is the best use of this model from my point of view.
If needing something kind of like the City Mini GT2 but smaller, check the TFK Dot 2.