You won't get faster burning speeds through the Ultrabase compared to an external USB DVD.
This covers the most important part of my question/comparison. Ultrabase DVD speed vs External DVD speed.
It surely is a lot more convenient to use a base which can actually be attached onto the laptop, something you can carry around, and still be lighter than a full size laptop.
If you have an i7 X220 with USB 3.0, I'd have to believe that would be faster than an SATA DVDRW drive... that is if you find/own a USB 3.0 DVDRW.
On an i5 X220 with USB 2.0, an external burner would be slower for sure.
I do not have the Thinkpad yet, this is going to be paired with a new X220, most likely i5 2.6 to save money. I also did not find any USB3 DVD writers, I think USB2 should be more than enough. Based on reviews at newegg, most of them are usable.
I say that the benefits of the Ultrabase become relevant if you want to sometimes take it with you on the road and have the expansion capabilities, or if you want to use a permanent second internal hard-drive.
Having another HDD is nice, but that would still be limited to when I have my laptop docked in the office. So any USB hdd would do the same thing via one of the USB ports of any dock. HDD would have to be just for backup because it will connect/disconnect, so it can't have anything important that I would use everyday when I am out.
It looks like the only difference between the two is really just portability option, and being able to include a drive in there. If I need battery, I would have to have it with my anyway, and I would rather not carry the dock with me, so I can just get 9cell or the battery slice later on. The add-on DVD is $140 with ultrabase, that's really expensive.
Thanks everyone, I think i will go with Mini Dock, whichever offers dual display option (two is enough).