ユーザ or ユーザー, which is more in use or is there no difference?
I see both of them in use, but is there one that is more correct than the other or is at least the prevailing standard?
a similar word is サーバ and サーバー.
From the point of view of the pronunciation,
ユーザー is closer to the original English word, and that is the generally accepted form in ordinary life, especially among people who are not particularly related to information technology. However, limited to the information technology context,
ユーザ is the form that is officially stated as the correct form, and official documents are recommended to be written as such. If you are interested in the detail, you may want to take a look at Japanese Industrial Standard Z8301 Amendment G Article 6.2c and Table G3. You can see a copy here: (logical pages 58 and 59).
Wikipedia (ja) has a nice summary about omission of chōonpu ('─') character in general:
(abridged translation by me)
From my personal impression, a person who uses ユーザ consistently sounds like he's been knee-deep in the industry for more than a decade, while ユーザー users belong to the generation Y who grew up living in the web.
However, this is an overgeneralization which I'll avoid until I'm pressed to make a judgment. In my own writing, I've used both depending on my mood, if it weren't specified by a prescribed manual of style.
Take whichever you want, but be consistent in your whole software/document/website.
My company often localizes Open Source applications to Japanese, and our rule is to not add ー if not necessary.
Also, Gentoo has its rule written like this:
Translation: If both variations are in use, then do not add the final ー
Since nobody has given any hard data on the question of which of ユーザー and ユーザ is "more in use", here is the result of the Balanced Corpus of Contemporary Written Japanese (accessed via http://nlb.ninjal.ac.jp/search/)
ユーザー 2566 results (84%)
ユーザ 487 results (16%)
For サーバー and サーバ the bias is less prevalent
サーバー 1011 results (55%)
サーバ 813 results (45%)