I just sold my HP Pre 3 this monday, and since then I'm back on the N900.

Although the Pre 3 is thinner and better in the hand and has better battery life, it has a lot of drawbacks compared to the N900.
Pre 3 has more apps, but most of them are shitty apps. And imo the developers on TMO are much more knowledgable. Even though Maemo has less apps, the apps are more versatile and of higher quality, and most are still supported.

Also there is this thing about Web OS, where it is hard to get your data out or in the ecosystem. You have to jump through hoops to export your contacts for instance. Yes, you can sync them to Google/Yahoo account. IF the sync works. Which it never did in my 9 months of owning the device. I never got my calendar data out. Much better to work with standard export/importing methods like on the N900.

And the camera is really bad. It wouldn't surprise me a bit if even the 3GS makes better pictures. The N900, which is 2 years older than Pre 3 takes much much better pictures.

The only thing that is much better on the Pre 3 is the keyboard feel. It has this sticky rubbery key. I always press the key with the tip of my thumbs. My fingers always slip on the N900 keyboard, specially when my nails are a bit bigger. Is there a way to make the keys on the N900 stickier?
But I like the look of the colored keys on the N900. Is much easier to to distinguish letters from the upper row numbers/symbols.
Oh, touch-to-type is something that really should be implemented on the N900. Is much better than hunting for an app, start it, and then type and send an email for instance.