This iMac Retina 5K 27-inch Late 2015 will drop out of its three-year AppleCare cover late this year, and is already starting to show its age. It will run Mojave, but it has only got 8 GB of memory, its 2 TB of storage is a Fusion Drive, and its graphics card is positively ancient, with a mere 2 GB of memory. It’s nearly time to let my son use it.

That was the easy decision to make. More difficult is working out which Mac to buy as its replacement. Although I’m not in any rush, and will probably not be upgrading until the autumn/fall, it’s wise to start planning ahead so that I know what to look for in Apple’s likely new models to be launched in September.

I want one 5K display, with a brisk i7 CPU, and 16 GB of memory. Unless Apple finally replaces the Mac mini with an attractive modular Mac, that gives me a choice of three basic models from the present range:

• iMac 27-inch, with either 4 GB or 8 GB graphics card,
• MacBook Pro 15-inch with an LG 5K display,
• iMac Pro.

There are ranges within those. For example, the graphics cards range from a Radeon Pro 560 to a Vega 56, the iMac Pro comes with 32 GB of memory and 8 cores rather than 4, but most of all the cost spans £/$/€ 2669 to 5619, according to those and the capacity of its internal SSD.

There are various pros and cons too. The MacBook Pro includes a Touch Bar and a built-in display, but isn’t built for constant running and is likely to have a shorter working life (compact laptops run hotter and are more expensive to repair). The iMac Pro has a good GPU and 8 cores, but little of the software that I use daily even makes the best of 4 cores, and eGPUs are going to come down in price, if they catch on at all.

The other major determinant of cost is the capacity of internal SSD, which could add well over £/$/€ 1000 to the total cost.

My current 2 TB Fusion Drive is just over half full, but mostly with static documents in my juggernaut of a Home folder. I have recently demonstrated how much quicker a factory-fitted internal SSD is compared with an external: four times faster when writing, and five times when reading. This is because an external SSD is likely to use 6 Gb/s SATA (that’s up to 0.75 GB/s), while Apple’s internal SSDs use 8.0 GT/s NVMe PCIe x4, which could theoretically deliver as much as 64 GB/s.

So I needed to look closely at the cost of SSD capacity, which is easiest using a graph. This shows, for each of the four models I am looking at, the total cost (inclusive of taxes) by capacity of internal SSD.

Numbers has already fitted linear regressions to those lines, which vary only slightly. The cost slope ranges from 0.72 £/$/€ per GB for the cheaper of the two iMacs to 0.738 for the iMac Pro. The overall equation works out at something like
cost = (0.725 x S) + M
where S is the capacity of the internal SSD, and M is a model constant of

• 2291 for the iMac with 4 GB graphics card
• 2420 for the iMac with 8 GB graphics card
• 3699 for the MacBook Pro + 5K display
• 4179 for the iMac Pro,

which are effectively the base prices in £/$/€ of those Macs without any internal storage.

Apple makes a handsome profit on its factory-fitted SSDs. In return for their better performance, they cost around £/$/€ 0.725 per GB. Typical slower external SSD storage costs around £/$/€ 0.20 per GB.

I think that an internal 512 GB SSD would be too small to cope with macOS, all my major apps, and the most active parts of my Home folder; 2 TB, on the other hand, would contain a lot of static documents, and be a poor investment.

The cheapest of the Macs with an internal 1 TB SSD is the iMac 27-inch with the poorer graphics card, at £/$/€ 3029. The better version of iMac, with a Radeon Pro 580 and 8 GB of graphics memory, is just over £/$/€ 100 dearer, but nearly £/$/€ 1300 cheaper than a MacBook Pro with 5K display, and more than £/$/€ 1700 cheaper than the iMac Pro. For the ‘exclusive’ keyboard of an iMac Pro, an extra 4 cores which would spend most of their time idle, and a faster GPU, that’s a huge price difference.

Clearly, my best value for money would be an iMac 27-inch with the better graphics card, and a 1 TB internal SSD. Your mileage will vary, and it will be interesting to see how this changes with Apple’s product launches later this year.

I’m still open to donations of an iMac Pro, of course.