Depends what you want to use it for. BTW what is the 'Classic' - The original TomTom Go?
As you already have a TT unit - you will be familiar with the interface - but are you going to use both in your car?
I would personally find the All-in-one solution more attractive. Having a separate BT GPS system is OK - but they need charging from time-to time. Also you will need to keep the Treo on charge in the car to stop it depleting the battery too much - you wil need to keep the backlight on to see it.
I do have an old copy of TT5 (UK maps) on my older Nokia 6680. It's works fine - but I find that navigating the app is fiddly - it's not touch screen. I tend to use it as a passenger in other vehicles. Phone/PDAs with touch screens are designed (by and large) to be used with a stylus - not something to be recommended whilst driving - and you are bound to want to display some other info at some point in a journey.