The problem with this line of reasoning is the 4,1 can be easily upgraded to match or exceed the specs of the 5,1 with the money saved from buying the 4,1. If he were buying a new Mac Mini or iMac future proofing might make sense.
Likewise buying the 4,1 Mac Pro is more future proofing than the 5,1. Why? Because it has a better processor. It has a higher clock speed and can Turbo Boost higher than the processor in the 5,1. Since the processor is, technically, not a user serviceable part (like the graphics card and hard disks) future proofing is better done with the 4,1.
Here's the situation:
The OP has stated integrated graphics are sufficient for his needs. He's further stated the hard disk configuration is, for the most part, irrelevant as he'll be installing SSD's. As such both the 4,1 and the 5,1 meet his needs as the graphics card and hard disks are, essentially, the only differences between the two.
So what if he should decide he needs more graphics capability? He takes the cost of a new graphics card out of his savings from buying the 4,1 and buys the equivalent (5770) or better graphics card. If he wishes to add more hard disk capacity he can, again, take the savings from buying the 4,1 and buy a $40 1TB hard disk. He is no worse off than had he bought the 5,1.
Given this I see zero downside to buying the 4,1 over the 5,1. Perhaps I'm missing something. If I am I'd appreciate it if you'd be so kind as to point it out.
Tell you what...how about a compromise. The OP says he want's two, one for him and one for his wife. How about he buys one of each, and then he can move the 5770 card into the 4,1 and have the best of both worlds?
He can give is wife the "newer" Mac Pro with the GT120, and everyone is happy!
I should add that, personally, I think the price for either is kind of high, at least compared to US$ prices. There are 4,1 Mac Pro's on ebay for less than $500 shipped.