I've not used any BT keyboards recently, so I can't recommend a particular one - it may be worth heading out to a PC store and giving a few a try, as it's good to get a feel of one before purchasing.
Bluetooth keyboards aren't 100% secure - so it would technically be possible to intercept... but it would be more secure than
some wireless keyboards. AFAIK, BT uses some form of 128-bit encryption - there's more mentioned on this Wikipedia article:
http://en.wikipedia.org/wiki/Bluetooth#Security
I did some googling on the logitech wireless keyboard range and it looks like they use 128-bit AES for the 2.4Ghz wireless range. Assuming that it's been implemented correctly, that should provide protection from casual hacking.
Personally, I'd be happy with either - but would probably go for Bluetooth for convenience. The range is pretty limited, so that also limits who could have access.