However, the MDN version is likely to not get any further upgrades that the ODN version will get. The ODN version already has features the MDN version lacks.
They will still update both versions of Navigator (MDN and ODN.) However, because it is newer, ODN is likely to get new features first. However, sometimes little surprises can show up in one division or box over the other.
For example, Brighthouse Networks has had MDN 2.5, for several months and they have a feature called DVR Compensation where you can control the level of jump-back speed when coming out of rewind or fast forward. AFAIK, I have not seen this feature on ODN. Also, Ben is exactly right, a true 16x9 widescreen HD guide, (IF TWC ever chooses to support it,) will only be found on the new C-boxes and Samsungs (ODN.) The same is true with muti-room DVR (ODN only.) This is because hardware limitations in the legacy boxes, currently prevent the activation of this feature on MDN.
Catch-up features will gradually be added to MDN boxes. All boxes will get Guide Filtering, expanded search options, and MDN is also supposed to get Nearest Tune. Widespread E-SATA support remains MDN's biggest selling point, with a box based on Navigator/Mystro's original Navigator concept. Some have reported that the SA-8240 HDC's and SA-8300 HDC's slower running time may be due to the newer OCAP (ODN), Cable-Card and Java Applet Overloads. (i.e, the box is overworked.)
If you want the newer box with the bigger hard drive, the Samsung 3270 and Cisco 8600HDC's with the faster processors may be the way to go. But if you don't need a bigger hard drive, like E-SATA working, and your MDN box works well, you might as well hang on to it. If you never plan on a second DVR, than multi-room DVR planned for the newer boxes is a moot point.
There's not enough new features concerning ODN to swap boxes just for that at this time. Now if Muti-Room DVR comes out and tons of new features get widespread to ODN, people might want to switch. However, that new MDN DVR compensation feature remains a mystery as to why ODN does not have it.
Oh, and eventually Remote DVR Scheduling is coming to all the boxes and because some divisions have reported that they only are carrying SA-8240 to SA-8300C DVR's, I think MDN will still be around for a few more years. TWC just doesn't want to throw out the older SA-HDC's, or SA-HD's, so we'll see what happens.