Choosing Thomas Cook's Heir?? (spoiler)

[Zhuangzi]

The diamond is worth 50,000 gold. I have heard that money isn't an issue on Medium so you might think twice about this. Also, you will need to make sure you have fully upgraded your barter skill (Mind tree) to get full value from this. As a Hard Paladin, I found that the diamond was very nice indeed. I always need the money. I chose the young brother as well and the Diamond is all I got.

[daydreamer]

I've chosen the eldest brother. That made the shops of all three brothers available to me. The eldest had a nice item if I remember correctly.
Also the elder brother said something about killing the other two brothers if you'd choose him, so I don't see how that would benefit you. Unless you would be the one carrying out the task (exp + gold reward from fights).