I'm lucky to be in the position where I can choose from both of these palm o/s devices and they both have their merits.
The Centro wins this hands down. It is very light compared to the 680 and is also considerably narrower and less wide. It really does feel like a normal phone in the hand, in the pocket you hardly notice it is there as it is so light. Very compact. The 680 shows its age in this respect and is bulky and heavy. It does feel humungous compared to the Centro but the upside of this is that it feels a lot more solid and hefty. It is pocketable but you are constantly reminded it is there.
I have to admit I have struggled with the Centro keypad. Being a smaller device brings compromises and it here that the Centro can struggle. I have large fingers and to use the keypad I have to be very precise with my pressing which after a bit becomes a strain and painful. I really don't like using it for longer typing spells and get to the point where I actively avoid having to reply to emails and stuff as for me, it is not a comfortable typing medium. I'm also not a fan of the rubbery coating or of the fact the keys are so close together.
The 680, being much larger, benefits by having a much better keyboard which is far easier to use for longer spells. I make far fewer typing errors on it and can type for longer without strain.the keys are spaced apart and are easier to hit accurately. I really think that if you use your palm for a lot of document editing/typing/email then 680 will be a better option.
Obviously the 680 has a bigger screen and this can be very handy when using certain types of apps such as Docstogo, browsing and any apps where there is a lot of text to display. It's handy having the extra screen estate but I'm not so sure it's a deal breaker against the Centro and would opine that the better keyboard is a bigger factor. The Centro screen is a bit brighter and crisper than on the 680, even with the brightness turned up on the latter, it appears more vibrant whereas that on the 680 looks a bit yellowish/greyish in comparison.
I know the 680 has the reputation of having iffy battery life but since the palm camera updater…errr…update it has been fine for me. An advantage of the bigger body is that you can squeeze in a bigger replacement battery. I use the Seido 1600 one and it gives me an easy 2 full days useage. I don't think there is an equivalent version for the Centro without delving into the replacement back cover territory. Nonetheless, battery life on my Centro is pretty darn good and gets me easily through a day with 30/40% to spare. The point I am trying to make here is that neither should be chosen over the other on the basis of battery life as in my view they are both good, especially if you can get the Seido battery for the 680.
I still find the Centro to be a tad quicker through the menus and in all round performance but it is not a huge improvement as both are still very quick devices, certainly in comparison to the likes of Win Mob devices. Both have ample inbuilt memory for storing applications. Both are SDHC compatible smartphones and therfore should support cards up to 16gb although I can't find proof of this on the internet in various forums, the Centro is certainly comfortable with at least 8gb micro SD.
Both run the same system, both are quick and both allow access to the same huge range of 3rd party apps so it is a draw here. However, the camera on the Centro (at 1.3mp) is much better than that on the 680 but that would not be difficult bearing in mind the weedy effort on the latter. If taking pics is your thing then the Centro is a far better option but the specs are still way behind the majority of other smartphone cameras.
To me the 680 is more of a solid unit than the Centro, less creaky too. I also find the buttons easier to use, the Centro's middle silver strip is a bit flexy and more care is needed to use the keys properly.
Virtually the same on both devices as the menu system is extremely similar but I find the call quality and volume on the Centro to be much better. It seems much crisper and clearer when on calls, the voice quality on the 680 appears a bit murky and muffled in comparison but for my needs it is acceptable. I would say that reception on the Centro is better too.
In true cop out fashion, I can't pick a winner as that is up to you. The only areas that the 680 truly shines over the Centro is in the keyboard ease of use and extra screen estate. Now, I know people who use the Centro and find its keyboard perfectly acceptable in which case if you are in the same group then the Centro is no brainer winner. Luckily, here in the UK you can try one out in stores such as PC World and John Lewis and I would suggest spending some time trying the keypad to see how you get on.
Myself, I continue to prefer the 680 which flies in the face of what I have been writing here! I do so because I much prefer its keyboard, the bigger screen and the more solid feel. However, every time I pick up my Centro I marvel at its compactness and feel that I am making a mistake continuing to favour the 680. That shows what a tough choice it can be.
If you have a 680 and are considering switching to a Centro I'd be more wary. If you are happyish with the extra bulk of the 680 then moving to the Centro should only be done if you want the smaller size and better camera, there's really nothing huge there to make a compelling switch decision specs wise over and above those two things. Personally, I would only move to a Centro if you are completely cheesed off with the sheer size of the 680, want a better camera and can handle the smaller more cramped keyboard. If you can then make the switch.