Trepstar versus Kunaki is a common comparison, and having used both I think I prefer Trepstar.
Kunaki are cheaper but can't get more than one DVD into a case, meaning a 2-disc set entails setting up and ordering two separate products. Trepstar can do multiple discs in one lot of packaging.
Shipping times should be similar but Trepstar seems to be faster. I prefer Kunaki's cellophane wrap as opposed to Trepstar's shrink-wrap, as I think it makes the product look slightly better.
Neither business purports to offer outstanding customer service but Trepstar at least respond quickly and without the anonymity you get from Kunaki.
I would recommend both companies, as it depends on what you want, but given the choice I am compelled to favour Trepstar. The customer service is much better, the shipping is quicker and they send instant tracking info for each order to every customer, which means you don't have to!
The fact that I'm paying a little extra is made up for by getting quite a lot extra in return so I'm happy to continue to use Trepstar for all of my disc-on-demand needs.