Linux is a clone, it dosen't have a Unix heritage.. OS X is fully posix and unix compliant
The fact that OS X is UNIX(tm) and Linux isn't may be true, and a fun pedant point, but has very little practical upshot.
UNIX(tm) is a trademark that you can licence from the Open Group for any operating system that passes their conformance tests. POSIX is an ISO standard for a cross-platform API and full POSIX-compliance requires certification (in the past, even Windows has had some level of POSIX compliance).
The only real reason that Linux
isn't UNIX and POSIX is that the licensing and conformance testing costs money and doesn't play nicely with the GPL license*. AFAIK individual Linux distributions could pay up and become UNIX(tm) if they wanted.
OSX uses BSD-derived tools in some cases where Linux favours GNU tools - but those GNU tools have been popular replacements/upgrades for the traditional BSD and SysV tools on "real" Unix systems since the 1980s, and OS X uses a ton of GNU tools (including, until recently, the GNU C/C++ compilers) anyway.
The main feature of "Unix heritage" since the early days has been multiple competing "flavours" of UNIX and Unix-like systems interspersed with the occasional lawsuit. To claim "Unix heritage" as some sort of pedigree is a joke. To claim that OS X is not a "clone" is, to say the least, debatable and, if that really worries you, there is always FreeBSD which is certainly no more a clone than OS X. In practice, the way you use, configure and manage Linux feels far closer to a traditional Unix system than OS X does.
However, to get back on topic:
Since OS X shares the same Unix heritage, could OS X also be used in the same way with the same development tools as linux?
Basically: yes - a lot of the Unix development tools are already installed on the Mac, and some more get installed when you add xcode. Or you can install one of the free package managers - homebrew, macports or fink - from which you can easily (if, sometimes, slowly) compile most of the usual suspects from the open source Linux/Unix world. You'll need to install XQuartz to run Linux/Unix GUI software.
However, whether that makes sense depends a bit what you mean by "development" and "development tools". If you want to develop "native" apps for either Mac OS or iOS, then Unix is pretty irrelevant - you'll use xCode. If you want to develop "native" apps for Windows then you should consider using Windows (MS Visual Studio isn't rubbish). If you want to develop cross-platform apps in something like Python or Java then a lot of the tools (e.g. Eclipse or NetBeans) are cross-platform anyway.
I think the sweet spot is web development for deployment to a Linux web host. Because its a Unix system, its easy to replicate something very close to the environment of the server on your Mac. All the technologies you might need (Apache, PHP, Perl, MySQL, PostgreSQL, node.js, Tomcat...) can be installed either by a package manager or using an app like MAMP. Actually, several of them are already installed by default, but not always the latest versions, and using MAMP or MacPorts avoids messing around with system files. You can get the best of both worlds: the OS X GUI for editing, all your favourite graphics software (and/or the MS/Adobe software you need to deal with the files people send you) - plus the same server technologies and file structure as your target.
The alternative approach, though, is to install Linux on a virtual machine using VirtualBox (free), Parallels or VMWare and do your Unix-y stuff on that. You can have separate VMs for different projects, choose the Linux distro that most closely matches your deployment target and use shared folders so that you can still use Mac-based graphics tools and editors.
*That and, as burne points out above, pedantically Linux is just the kernel and not the operating system, although that is at odds with the way the word is used outside of the Linux developer community.
