Another possibility would be dual-licensing with CC-BY-SA and GFDL, but that's probably not worth the extra work unless you've identified materials you want to use that are under GFDL.
If the things they want to use are GFDL-only and the product is an adaptation of those then they don't have the option of dual licensing. You may have meant "you want your work to be incorporated into that are under GFDL."
Only do CC-BY if you simply want to make a gift to the world, and you don't care if your work is repackaged into something non-free by other people.
CC-BY isn't quite a pure gift -- it could be used by a selfish licensor if that person only cares about maximizing the amount of credit they get -- incorporating CC-BY works into non-free works still requires giving credit.
The GFDL and CC-BY are rather different licenses. The first is a copyleft license (requires adaptations to be distributed under the same license), the latter is a permissive license (do anything you want so long as you give credit, roughly).
If you don't want copyleft, CC-BY is your choice.
If you do want copyleft, it would make sense to choose between GFDL and CC-BY-SA, which you can think of as the copyleft version of CC-BY. Wikipedia (and other Wikimedia sites) migrated from the GFDL to CC-BY-SA as their primary content license in June, see http://creativecommons.org/weblog/entry/15411
Thanks for not considering a more restrictive license.
He has not acquired a fortune; the fortune has acquired him. -- Bion
