Both ubuntu and debian are heavily used probably in the making of the maemo platform so essentially choosing those over <insert_your_own> distribution would be wise in various ways:
- Guides on maemo.org are mostly written for either of these distributions.
- Scratchbox developer environment for maemo has already been made in virtual machine format.
- Essentially the core of the package management system used in maemo is deb packages which are debian package.

Choosing ubuntu over debian would be somewhat a wise choice for a beginner to linux as ubuntu is more lenient towards beginners than debian. However nobody says you are forced to use ubuntu

As for using virtualbox or any virtualised environment, there are some limitations:
- You can only allow raw/direct access of USB based hardware to a guest OS. Any other raw/direct access are not possible with the usual virtualisation software.
- Using virtualisation means that you have to sacrifice a portion of your system's current running resources in order to have more than one OS operating at the same time. The guest OS needs to be allocated a certain set of RAM (which can be a little daunting for linux beginners assuming the bare minimum is enough). Also not everyone has enough system resources to run virtual machines, people with maybe 1-2GB of RAM may struggle here when they try to expand their usage.
- With a virtualised linux environment, you won't be able to get access to raw terminals or even magic sysrq key(for when you're stuck in a bind and you want to quickly finish it up). Most linux distributions comes with at least seven terminals, the first six consists of only pure CLI based terminal whilst the last terminal is usually for the Xorg environment (which is GUI in layman's terms).
- I also personally find that having linux virtualised and with windows being the host, one can find themselves disturbed and willing to switch back to the host. With dualboot, there's no switching back and you're forced in that environment until you either bail out or whatever. This is especially handy if you really want to learn linux properly rather than having linux as a guest OS.