If you go 40mm, the HLBS figs are great, but there are also 40mm old west figures at Knuckleduster.com. Knuckleduster does something similar to Old GLory in that there are foot and mounted versions of many of the figures, but they are sold as separate packs; So you'd get three foot figs in one pack and three mounted figs in the other.
Between the two companies, you also have a great selection of townsfolk. I also like that Knuckleduster has casualty/wounded figures (men AND horses.) HLBS, Knuckleduster and Sash&Saber are compatible. Sash & Saber, by the way, has a beautiful 40mm scale stagecoach at a decent price.
The only drawback I found to 40mm was the lack (or at least shortage) of buildings, which is why I went ahead and built my own (that was my blog mentioned earlier.)
If you choose to go 28mm, nothing wrong with that!

You'll have hundreds of figures to choose from. And the Arnica buildings over at Hawgleg are about ready to go (if they haven't already started shipping)