I don't think there's any single rule of thumb that applies to all teas.
Factors like steeping time, temperature, types of leaf used, and processing can all make a difference. Studies that look at the dry leaf
will usually say that they're roughly equal across all types of tea. Ones that look at the final infusion
will generally show a step up in caffeine by the level of oxidation.
Take a look at this article - http://lpi.oregonstate.edu/infocenter/p ... index.html
I tend to think that the more "oxidized" a tea is, the more readily the caffeine will extract into the infusion. There are some exceptions, however, such as silver needle, which seems to have a relatively
high level of caffeine.
The difference is easy to understand when you know what "oxidation" entails; to oxidize a tea they bruise the leaf, breaking the cell walls and bringing the juices to the outside of the leaf, and then dried. Obviously this will leave all the chemicals that were previously on the inside the leaf much more readily available for extraction to the final infusion. If you then use hotter water and steep for longer, it's going to have an easier time getting what's left.
Conversely when the chemicals are bound up in the fiberous material of the leaf, and the leaf is only dried without much more processing of any kind, and then steeped in cooler water for shorter periods of time, then you're not as likely
to get as much in the final infusion.
So the bottom line is that my experience, and research on the subject, seems to suggest that most
white teas produce an infusion with less caffeine, and the darker teas tend to produce infusions with more caffeine - even if they dry leaf has the same approximate amount of caffeine. There are exceptions both ways in all classes of tea, but this seems to be the over-generalized rule of thumb that I have come across.
If you have questions about a specific tea, I am sure that people here would be happy to weigh in with their experience with it