Ok I dont quite get this the new MEPIS is based off of Ubuntu 6.06 LTS(long term support) and MEPIS wants to offer the same kind of services. Now this is the thing MEPIS cost money (simplyMEPIS is the free livecd version) so why would you buy MEPIS if it is based off of a free distro like Ubuntu. So simplyMEPIS is based off of Ubuntu which is based off of Debian.
The difference between the two distros is the packages that come with it. Ubuntu does not come with preinstalled proprietary software (non opensource), but MEPIS does offer that software. So things like audio and video codecs and that sort of stuff come preinstalled.
I dont have much to say about this there have been posts on both multiple linux forums about which one is better. In my opion Ubuntu is just because there are so many users and so many people who contribute documentation and information to the community. I just dont really see the point in getting simplyMEPIS if Ubuntu is the same thing with a larger community. Especially if the only difference is a few codecs that scripts like EasyUbuntu and Automatix could install in a couple seconds.