From the Ubuntu NewPackages wiki:
Submitting new packages through Debian is the preferred path. But, if your package is Ubuntu-specific or can't go into Debian for some other reason, you can submit it directly to MOTU. There are a limited number of available reviewers, so you may encounter delays here.
So, you should consider packaging your software for Debian Sid (the Debian continuous "development" version) and then, once this is accepted, ask for your package to be copied into Ubuntu repositories (this is called syncing, see the Ubuntu for Debian developers page).
Practically speaking, and from my own experience, you will have huge difficulties to find a sponsor to review and include your package in Ubuntu directly unless you are really working on some Ubuntu-specific software. It also makes more sense to include your software in Debian as your package will be synced to all other Debian-derived distributions besides Ubuntu. Finding a sponsor in this context will be much easier.
If you want to go through MOTU (upload into Ubuntu directly), use the current Ubuntu development version here. At the time this post is written (Tue Apr 30), it is Ubuntu 13.10.
On the opposite, if your goal is not to have a package officially included in the distribution, you may consider using an already released Ubuntu distribution. Usually, in this case, building a Debian package is mainly to avoid configuring and recompiling the same software on a set of computers your own. In this case, just choose the distribution these computers are running as an Ubuntu package for 13.04 will not probably be installable on an older release.