I'll start by saying that I'm not a Triton guy.
But I assume that you're talking about a slimline featherbed frame. If so, I have some experience in that category. I have an Atlas special that I built several years ago, and was in your same situation when deciding on wheel sizes.
My main preference would be to run a 19" front and rear. But at the time, I opted for a 19" front and 18" rear.
I run a WM2 19 on the front with an Avon Roadrider 100/90 - 19 tire
I run a WM3 18 on the rear with an Avon Roadrider 110/90 - 18 tire
I always use those Avon tires, and I personally think they're excellent for a classic road bike.
Those 2 tires are nearly identical in overall height. Which was my goal. To have a balanced look in ride height in conjunction with the wider rear tire.
I can tell you that on a stock slimline featherbed swingarm, the clearance for the 110/90 - 18 is very tight. Once laced up (by Buchanan's), I had to adjust in order to move the rim ever so slightly towards the drive side. And even with that, the clearance is tight on both sides. About 1/16" give or take. So you spacing is important. And obviously the chain length will determine where the tire sits within the swingarm. I have some room to move my tire back which would give a bit more side to side clearance as it gets away from the taper of the swingarm towards the front. But I've settled on it's location since the drive side of the tire would have less clearance if I move it back any further. This is because I still have the welded on stock chain guard mount. So it's a bit of a compromise all around.
In the end, it's all a matter of preference I suppose. I don't think my wider rear tire selection gave me any added performance. If I were to do it all again, I'd go with WM2 19 front and rear with 100/90 - 19 Avon Rpadriders. And again, that's just preference. I like the stance of my bike to look balanced. And while it looks very balanced with the current set up, I still like the look of both 19" wheels.
Depending on your tire choice, I'd check with the maker for actual measurements. For instance, my 110 wide rear tire is listed as 116 actual width on the Avon site.
Based on the measurements, I believe that a 110/90 - 18 would fall in the swingarm at almost exactly the same spot as a 100/09 - 19. Giving the same clearance that I currently have.
All of this also depends on your shock length and also possibly your center stand. I've heard some say that with the larger tires, the bike would be unstable on the center stand. I've never had that problem, but figured I'd note it. You could always weld some thin metal pieces onto the bottom of the center stand feet if you encounter that problem.
Hope this info helps. Again, this is just my personal opinion and personal experience.