Every iPhone release is actually a significant upgrade if you look into the improvements Apple made to the SoC every year. So internally, I think every year is a significant upgrade.
Number cycle pros:
- new physical design
- design last you for 2 years
Cons:
- may have physical issues (4: antenna gate, 5: durability of the chamfered edge/scratches, 6: bendgate) that are not immediately known in internal testing
S-cycle pros:
- addressing issues of the physical design
Cons:
- your phone will look old within a year
I mostly agree, although it does seem like the S-cycle devices typically get a bigger speed boost than the Numbered-cycle devices.
The one exception is the iPhone 5. A6 and 1GB of RAM was a massive jump over the A5 w' 512MB. I don't think the transition from A7 to A8 was quite as dramatic.