MacOS – 2 Column Text plain text or markdown editor

Have you considered Xcode?

I know you said you don't like TextMate, but it's one that meet pretty much all your needs.

which is fast

TextMate
Really fast

Xcode
Not that fast but not Eclipse/Netbeans-slow

which does syntax highlighting

TextMate
Very complete and extensible

Xcode
Does syntax highlighting, it depends on your needs

were I can define own syntaxes (or there are many plugins e.g. for YAML),

TextMate
Very, very extensible

Xcode
Syntax extensions are possible

where the developers are active,

TextMate
Some say it's vaporware, yet very few other text editor can compete with its current features

Xcode
Apple development of this program is very active

which does not have the look & feeling of TextWrangler

TextMate
If you don't like the sliding bar, there's a plugin to replace it by a drawer

Xcode
Doesn't have a sliding bar

which can define projects (like Coda can)

TextMate / Xcode
Do projects

which includes a basic support for Git and/or SVN

TextMate
Support for Git/SVN/Mercurial (and probaby more) via built-in and added plugins

Xcode
SVN support for 3.x, added Git in Xcode 4

which supports a fast autocompletion

TextMate
Basic variable/function completion
very complete code completion via bundles

Xcode
Basic variable/function completion

MacOS X 10.6 compatible

TextMate
Some compatibility issue

Xcode
Fully compatible

You might want to take a look at Michael Tsai's iPhone text editor's comparison. It's not specifically for iPad, but summarizes (and he keeps it more or less updated). You have links to all the apps he mentions and a nice formatted table with the "features" each editor has or lacks.

He specifically added iPad support in the linked version of the entry:

I’ve added rows for tracking iPad

And of course, at the time of writing this…

WriteUp, still my preferred editor, syncs faster with Dropbox than before and now supports .taskpaper files.

_{Bear in mind that Michael updates the entry every now and then. I've linked to version six, but for the sake of this AskExchange entry, you should check if there aren't new ones. Use a google search like this. If you don't know what google is, you can try this other link. ;)}