I’m not a Ubuntu user, but from what I have read, the biggest difference is likely the Software Package Management where Ubuntu’s is .deb based while openSUSE’s is .rpm based. There are good and bad points to each, but IMHO they are roughly equivalent, and once you get to know one well, the learning curve to learn the other can be discouraging sometimes.
Another difference is openSUSE is sponsored by Novell, and as such openSUSE is sort of a front end to SLED (SuSE Linux Enterprise Desktop) and SLES (SuSE Linux Enterprise Server) packages of Novell. And also as such openSUSE has a relatively short support life (only 18 months), while I think Ubuntu may have versions with slightly longer life. It also means openSUSE provides the capability to focus more on the Server side than I think Ubuntu has.
Also because Novell sponsor openSUSE, and because Novell have a massive amount of cash in the bank, SuSE-GmbH (owned by Novell) have to be much more careful in how they link to packages of proprietary codecs … while Ubuntu, which is a corporation supported by a Millionaire (who I think has legally distanced himself from lawsuits on Ubuntu) has less worries for proprietary codecs, as Ubuntu is still losing money (with a negative cash flow). ie no one is going to sue a company with no money. But people WILL sue a company with lots of money in the bank. This means Ubuntu can and they tend to take more legal risks with the ease of install of the questionably legal codecs for proprietary multimedia, than what Novell (with openSUSE) or Red Hat (with Fedora) are inclined to do (in terms of taking risks).
The two distributions also have a different emphasis on desktops, with Ubuntu more focussed on providing a polished Gnome, while SuSE-GmbH tends to split its efforts between both KDE and Gnome (possibly with KDE getting a slight edge).
Because of its longer history (going back BEFORE there were forums) openSUSE’s support tends to be significantly more fragmented than Ubuntu’s, as openSUSE has the mailing lists (in addition to the forums) where the packagers tend to hang out more on the Mailing Lists than the forums. Thus sometimes a better sence of openSUSE’s direction can be obtained from a Mailing List, while in the case of Ubuntu the forums have more importance.
OpenSUSE with its packaging by SuSE-GmbH and support by Novell tends in my view to be SIGNFICANTLY faster (in general - there are exceptions) to passing fixes to Linux upstream so that the developers will incorporate fixes and ALL Linux distributions will benefit (from openSUSE fixes). Ubuntu, on the other hand, often sends their fixes to Debian, where because of the volume, can often languish and may or may not make it upstream. Hence IMHO other Linux distributions do not benefit as much, nor in a timely manner, from any fixes Ubuntu comes up with, while I believe other distributions benefit much more from openSUSE’s packagers efforts.
Thats all a very personal view, and you can be ASSURED that others will have a different view.