The answer for your first question ("not seen neihter") I would have say "the anime". Just because hardly a drama based on smth will be better than the original. Esp if the drama has only a few episodes.
For the second question ("how much worse is the drama" and "if one can enjoy it without having watched anime"). At first I've seen the drama some time ago and it was so-so. I actually have forgotten about itXD Recently I've read the manga (not really liking Mafuyu's puppy features at first but got used to it) which was pretty interesting and enjoyable; fun fact: I've found the relationships other than Mafuyu x Uenoyama better and more compelling.
And while reading I saw the anime. It was great. I got the shivers listening to "Fuyu no hanashi". While listening to "Melody lane" (Mafuyu's song from the drama) I had not. I think the cast could have been improved also.
Another fun fact. After enjoying the manga and the anime and recalling that I've seen the drama I was surprised that I gave it only 5. So I rewachted, it's pretty short. My rating rose to 6-6,5 but still: manga>anime>drama. [Just note that the anime is only a little less or equal to the manga. I guess that's because "Given" is the story with a lot of music and music is very important factor. Sadly a manga cannot convey it.]
