That depends on how much you're going to use the PDA functionality of the device.
As with anything in life, single function devices are usually better at what they're meant to do than multi-function devices.
A PDA phone will give you lots of PDA functionality, but it won't match a regular phone when it comes to making calls and sending texts.
Q. Are you going to be using the PDA functionality a lot?
A. Everyday and all the time! Go for something like the HTC Universal* or XDA IIs*. They make great PDAs and offer a fast CPU, but are very large and often the call quality is poor. Texting with the Pocket PC operating system they employ can be a very fiddly affair.
A. Quite a lot, maybe every day Go for something like the Sony Ericsson P910 or i-mate JAM*. They offer you almost all of the PDA functionality of the devices above, but in a pocketable form-factor. Making calls and sending texts on the P910 is pretty easy, though still not as good as a regular phone.
A. Quite often, mainly music and syncing my contacts and calender Go for a smartphone like the Nokia N70, Nokia 6680 or Orange C550*. They offer a good mix between size, style, PDA functionality and phone functionality. They don't offer the touchscreens or processor power of a full-on PDA, but are almost as easy to use as a regular phone.
A. Maybe a tiny bit, but not often. Go for a regular phone and enjoy the simplicity and size it offers you.
* All these phones are made by a Tawainse company called HTC and rebranded under different names. You'll probably find that a few of the operators sell them under a different name.