I'd like to get some high-res photos printed (on 18x24 and 30x40), but from the photos I've seen of the end result, they seem lower resolution than printing on poster paper. Is this true or is it just my eye?
If you refer to canvas as cloth, and paper as a glossy paper, yes.
The final resolution a printer can deliver depends a lot of the quality of the material you will be printing on.
This is mainly becouse an effect called "dot gain", which is how much a dot of ink (mainly liquid) spreads on the fibers below it.
If you have a tiny droplet of 5 (lets say) microns, and the fibers suck that ink 5 more microns your dot gain is 100%.
But if your minimum dot is 50 microns, the same absorption of 5 is only 10%. So on an absorbent material you choose to have bigger droplets so you can have more control.
As small details will not be resolved, you could use a lower resolution on your image.
On a glossy paper (or coated one) the paper has a coating that prevents the paper to suck the ink, so this droplet stays in place.
Let us say that you could use a 200ppi image on a coated surface and 100ppi on a canvas.
This all really depends on how you are printing and what you are using. Printing on canvas is a lot like printing on paper; you have to follow it up by knowing what kind you are printing on. Similar to how you will get difference results printing with cheap ink on xerox paper than on a glossy photo paper, canvas prints vary on what you are using. Depending on the treatment, fiber count, and coating, you will get different results.
Source: I actually run a canvas printing service qcanvasprints.com and getting this to stop happening was one of the challenges of getting our printing process set up.