The above users has perfectly summed up the pros and cons of both cities perfectly. I think Ahmedabad may offer better options for you especially with international city infrastructure. Surat on the other hand might be too cramped with confined space. Being the capital city, Ahmedabad and its periphery like Gandhinagar are going to be the epitome of new development. Modern age bus transit system and Ahmedabad- Gandhinagar metro once functional is going to make Ahmedabad as one of the best modern day cities of India.
Some of the popular projects in the city of Ahmedabad worth investing in are Shantigram of Adani group, Godrej Garden City by Godrej properties, Applewoods of the Sandesh group and Smile City of the Ganesh Group.
So if you are associated with some business establishment or would need to travel to Mumbai, Surat is a better choice. Surat does not have an international airport which is located in Ahmedabad. If you are just focused on the returns, Ahmedabad being the central hub offers a slightly getter returns in medium to long term. You can check out properties in Gandhinagar area or western Ahmedabad in Satellite region.
Good to know that you want to invest in Gujarat after living overseas. Both Surat and Ahmedabad are two of the most vibrant cities of the state. Both the cities have their plus points and negative points so it depends on personal choice. Ahmedabad is more of a cosmopolitan city while Surat is traditional at its core. Surat while being closer to Mumbai is a hub for trading especially in the jewelry and apparel segment