Ultimately people need to get into the idea of a separation of the hardware and the software (something that is more difficult with Mac as Apple restrict hardware substantially).
I recently moved my Wife from a Mac to Windows 8.1 after her iMac hardware began to creek under her Animation workload (though like me she also does photo editing work).
Though she loved her iMac and the screen was great the Core 2 Duo and 4GB of RAM was really beginning to struggle. This left us with 3 options:
1. Another Mac (either high end iMac or Mac Pro)
I did seriously look at this but the cost to spec a 27" iMac to have a i5 Quad with 16GB of RAM pushed it towards £1700. With a Mac Pro being more expensive still. Effectively paying £500+ for OSX struck us as too steep.
2. A Hackintosh
This is a bit of a wildcard option. I did run one for a little while but you have to accept some things may not work as expected or just be completely non functional. It also has questionable legal status.
3. A Custom Built PC
Fortunately with me working in IT having built several machines I was quite comfortable about this. I was able to design and build something from the ground up for her requirements which included:
Quad Core i5
16GB RAM
120GB SSD for OS/Apps
2x 1TB HDD in RAID 0 for Data (this is backed up over a network for redundancy)
Nvidia 750Ti Graphics Card for GPU Workloads
A custom case of her choosing
High Quality Dell Monitor
I very much agree with arad85's statement above about a £350 Windows based Laptop not being able to compete with a £2k Mac.
These days I would argue the biggest difference anyone can make to their machine is swapping out a HDD for an SSD or in the case of a tower running them both in parallel.
If I was going to be designing a workflow for a professional photographer regardless of operating system (which in my opinion is more of a usability thing) the hardware has to be appropriate.
If they wanted an OSX based laptop I would probably suggest the cheapest MacBook with 256GB of SSD and 8GB of RAM configured (which I think is still likely to cost ~£1k)
If they wanted Windows I would suggest something like this
Dell laptop.
I would pair both the above options with some sort of high capacity, redundant NAS system to sit at home with.
If it was a desktop I would personally recommend Windows every time from a cost saving a customization perspective and get someone like Scan to build it for me. If it has to be OSX I might be tempted with a Hackintosh. Interestingly I have seen people build hackintosh builds because the top end Mac Pro's aren't quick enough for specific applications. Again this is of questionable legal status.