Better to take gold or experience in Might & Magic Heroes VI?

It depends.

Generally speaking, a larger army is better than a few levels on a hero, so that would indicate that it is better to take the gold. However, as your hero gains levels, it takes more and more experience to gain a level, which makes the experience in the treasure chests worth relatively less, which indicates that you should take the experience to maximize its effectiveness.

Therefore, I usually choose gold on a small / medium map (when I know I will encounter enemy heroes in a short timeframe) and experience on larger maps (when I know I will have time to explore and build out my main town prior to encountering the enemy).

In the late game, you usually have more than enough gold and a high level hero, which is when I start leaving the treasure chests so that secondary heroes can get a few easy levels.

Because your hero carries over from one campaign level to the next, and because I've never been limited by gold at least so far, I've always been taking experience in the campaign. If you need a boost of gold to accomplish some goal, then by all means take it, but I would only give up the potential experience if you're going to put the gold to an immediate use. It takes a lot of experience to level up as you get further in the game, and one extra level up can mean a very powerful new skill.

2000 gold or 1500 experience - I take experience. 1000 gold or 500 experience - I take gold. 1500 gold or 1000 experience - it depends but usually gold.