You can change how the two keyboards come up in text-input settings. The larger one is likely set to open on "finger" touch, where it comes up if your n810 thinks you are selecting a text box with a fingertip rather than a stylus. As far as a better keyboard, nope, none exist (AFAIK). There is a matchbox keyboard installed with easy-debian that can be toggled on and off, but it isn't much of an improvement over the default stylus keyboard. Opera Mobile isn't open source, and there is little incentive for the company to open it up, so don't expect their keyboard (which runs on their proprietary UI framework) to be made available separately anytime soon.

So your options are
1. Learn to live with your current setup or the hardware keyboard
2. Set the larger keyboard to come up on "stylus" touch as well and make more use of the screen real-estate by enabling word prediction
3. Code your own keyboard
4. Only use Opera Mobile