bitcoin-qt because it is always the safest keeping money yourself, just don't let hacker took it..
The core Bitcoin-QT is relatively safe if you encrypt the wallet and are careful to avoid getting malware/trojans onto your machine. If you are not careful then there's no more security having your keys in that file and accessible to whoever can access your backdoor.
You would need to keep the wallet offline if you're sloppy and download all kinds of "junk"
Yeah, but I think the biggest caveat of BitcoinQT is still that it has to download the whole freaking Blockchain! It's just too big nowadays. Therefore I really do prefer a lightweight client like MultiBit...