Just something to remember, Apple displays are pretty top notch when it comes to using them for Photoshop, et al. An iMac or an Apple laptop will have a better display than a PC with a cheaper display, or just about any PC laptop.
That said, I use a homebuilt PC. For about $1300 I got a machine equivalent to a very high end mac pro. Add a few hundred more for an IPS display, and I have a machine that does a pretty good job in post. On the flip side, I deal with a much less reliable operating system, and despite some incredible horsepower (4GHz quad core, 16GB of RAM, SSD, etc.) I still have to deal with occasional hiccups that you wouldn't have on a mac. OS X is based on rock-solid unix, it's just a better operating system. Unfortunately, Apple doesn't allow you to install OS X on a PC. It CAN be done, but it's such a complicated process that yields unreliable results with, obviously NO support from Apple (including no updates) that, in my opinion, makes it no more than a novelty and no better than just running Windoze.
There's a lot to weight. With a $1300 budget, I don't think either would be a bad choice. You can't look at an image and say 'ooh, that one must have been done on a mac', unless the comparison was one done on a macbook vs one done on a PC laptop with a crummy inaccurate display!
If this were a discussion about laptops the display alone would lean me to suggest the Macbook, but when it comes to desktops, I think both are valid. Good luck!
Also, if you go the Mac route, don't discount the Mac mini. It has very similar specs to the iMac, but you can pair it with a good IPS display and come out a little cheaper, FWIW. Pair it with an Apple Cinema Display (one of the better displays out there, with an incredibly high resolution which is really nice for working on high resolution images), and you're only a couple hundred bucks over your budget, something to think about. (Or a $400 PC and an Apple Cinema display)
So, in my humble opinion, a decent multi-core processor and a decent amount of memory will handle the latest photo editing software. Your budget should
REALLY consider an excellent display, as that is a crucial component of photo editing. Dell, Apple, and Hewlett-Packard all make fantastic IPS displays for this kind of work. Though I'm partial to the Apple and certain Dell offerings because of their very high resolution (2560x1440). When you 'zoom out' on an image you are working on, there is a level of distortion and a level of inaccuracy to the color because several pixels are now crammed into one. A high resolution monitor allows you to view the photo at 100% (pixel for pixel) while still retaining a workable size, AND the color accuracy of a good IPS display really improves your end result. I have an IPS display and a second monitor (NOT used for photo editing outside of a place to stick the tools windows or the 'workflow' in lightroom) that is an older HP model, and the difference in color is incredible... makes me wonder how people get away with editing photos on something like that!
So if it were me, the options would be;
An iMac for $1300,
A mac mini and $1000 for a display,
Or $1000 for a Display, and $300~$500 on a decent PC (that would be, spec wise, in line with the iMac anyway)
I think ANY of those three would be a good combination.
You
DO NO have to just like one or the other. Both Macs and PCs are excellent machines that can do any work you'd like, each with their unique advantages and their unique disadvantages. I've owned both all my life, at the same time, and at different times.
and it's not like you can't get in and really muck about if you really wanted to. While Apple has an obnoxious habit of obscuring things from it's users, it's all right there if you know how to and need to access it (or just want to destroy file systems). Many windows users complain that you can't access underlying systems on a mac, and that simply isn't true.
You can futz up a Mac just as much as you can a PC, it's just a matter of knowing how.
In fact I prefer Mac over Windows in this area as Apple doesn't tend to spew shared libraries all over everywhere.
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But as pro mac as I am, the ONLY reason why apple is the choice for "creative professionals" is because Adobe used to be a Mac-only developer. As such, PostScript was only available on mac. PostScript is what made everything in desktop publishing possible in the early 1990's, from laser printers to digital typesetters. It really has nothing else to do with anything other than tradition. This tradition has allowed apple to market to creative professionals, and implement features which creative professionals might appreciate. But in the end, it's just tradition, and nothing else.
+1. In the 90's Apple was the creative professional choice. Period. Bar none. No exceptions. If you weren't using an Apple computer, you were severely limiting yourself. It's like "professional photographers" using a point and shoot camera. Do they get pictures? Yes. But they are severely limited and the quality of their product is sub par.
Now, however, you can do all of it on the PC. But many have never felt a need to shift over, and many people who make a living as a graphics professional on a computer have no desire to learn a new interface or learn how to tackle new challenges. They, in turn, teach the next generation, and so on. So Apple remains the most-used system for creative professionals. That's not ALWAYS the case though. I've got a relative who is a creative professional, does marketing/graphics design/photo retouching (for ads and articles) for a large firm. He now runs a PC with 3 30" high-end calibrated IPS displays. The PC has the software he needs and runs a few specialty pieces of software not available on Mac OS. He has worked there for close to 30 years doing this same job, and as long as there has been a Macintosh (just about), he has been using it, all up until now. Does that mean PC is better? No. Does that mean Mac is better? No. It means the two have converged to a point that either one can be a winning combination in capable of hands, whichever has the software availability and features you need is the one you should choose!