Also, the Avon lists the AM26 both ways; as 90/90-19 and 3.25-19. Several sites have them both listed, as if they're different tires. Is the cross section really different? Would Avon do that?
The real question is what are you going to do with it?
Standard road travel nothing wacky - I'd stick with Battleaxes, pretty solid tire.
I was looking at the Spitfire, too.
I have the Avon Roadriders in both 3.50-19 and 4.00-18, and 100/90-19 and 120-90/18. The former are on the bike now (CB750), the latter are headed for it.
The "inch" size ones are the same tire carcass and round-wrap profile core as the metric ones. The outer tread on the inch versions are merely a different thickness and width, and don't wrap around quite as far as the metric ones. In other words, they just LOOK inch-y, and are slightly smaller, in between the 100/90-19 and 110/90-19 (sort of like a 105/90-19) for the front one, and in between 110-90/18 and 120-90/18 for the rear one (sort of like a 115/90-18). Other than this apparent half-step size, they appear to be the same type tire and build.
However - they are slightly undersize in height, compared to their REAL inch cousins, which I have an old pair of (3.25x19 and 4.00x18 Bridgestones) on some spare wheels: the inch ones are taller by about 1/4" overall. This affected the handling on my 750 enough that I sprung for new 100/90 front and 1290/90 rear to get back the more solid 'line' in corners under power.
But, as some here have noted - I am PICKY... 
Well now... I always thought the inch sizes, (3.25 X 19 and 4.00 X 18) would make the bike handle better as they were the OEM sizes. Mark, are you saying you prefer the handling of the metric sizes? If so, I may have to rethink my choices for my new build... or am I missing something?
While I like(d) the inch sizes better, because their roll-attack angle profiles match the chassis, the modern inch sizes are not the same profile, nor is their construction the same. For one [big] thing, the bias-wrap design of the metric tires is at a different angle from the old type, so they stand taller than wide, even when sold as inch sizes, as the tire molds are all the same metric type. The one exception to this was the now-long-gone trigonometric tire (Dunlop K81 TT100 from UK, not Japan), which was a vee profile done in the same bias angles as the modern metrics, but with a narrow flat middle and wide, slanted sides that laid a bigger patch on the road when heeled over than when upright. These were expensive tires, born of roadracing and modified to use street-rubber compounds. Interestingly, this (my old favorites) made a better grip in the wet both upright and cornered, as upright it had a large center groove to let water out of the patch, and heeled over had large blocked grooves with a wider contact surface. To get this sort of performance in the metrics, we have to run considerably higher tire pressures to ensure the patch does not roll under when cornering with the bias arrangement they all use today (except those silly touring radials for Gold Wings and Harleys that don't corner so much - they roll under anyway!).
To make my Roadriders grip well when rolling side-to-side, I run about 38 PSI in the front and near 40 in the rear (1-up riding) with the 3.50 front and 4.00 rear. When doing it with the 100/90 front and 120/90 rear, I run equal pressure both ends. When I ran the 110/90 rear with the 100/90 front, I had +2 in the rear tire to make it "stay straight" when heeling over and then coming back up, with 36 PI up front and 38 PSI in the back. This wasn't enough: those tires wore in their centers too quickly, indicating the overall PSI was on the low side, but at least they flexed evenly with the 2 PSI difference.
