HO vs O.
Tell us about what you'd like to to for building your layout.
Do you have a nice garden outback in which you might want to build your layout?
Do you have a large or small space inside?
Do you care about scenery?
Do you care about figures?
Do you have a large or small budget?
Do you like to build your own models...or just drop it on the track?
There are a few different O scales. The American definition is 1:48 scale...and std gauge track scales to 5' gauge. If you go to Europe, it is 1:43 and scales correctly to 56.5" gauge (HO is derived from European O scale...87/2=43). There are two common types of std gauge O scale... 3 rail and 2 rail. There are two basic levels of 3-rail...less detailed (you may consider it toy trains) and super detailed (definitely models). I've never met a 3-railer whom wasn't convinced that he was sitting on a gold mine...and I can't recall meeting a non-3-railer who agreed with them. There are 4-4-0s available in O-scale.
O scale narrow gauge can be divided into a bunch of different groups, but I'll focus on two: On30 and On3. On30 has become very popular in the past ten years thanks to Bachmann. It operates on HO track. In US history, 30" gauge practically didn't exist...something like 40miles in the entire country...but, the track can be cheap (unless the ties bother you...and they seem to bother most On30ers that have layouts). On3 is very different from On30, 2-rail, and 3-rail. The track scales to an accurate 3' which was by far the most common narrow gauge. Until recent years, you had to be a craftsman to model in this scale. That has changed. You have to mail order everything, but it is excellent for the wild west thanks to MMI (Precision Scale Co)'s 4-4-0s and Accucrafts 2-8-0...both of which are being offered in both On30 and On3. I know a few On30ers whom have switched to On3 as the rail spacing of On30 drove them nuts. The trade off is that your min curve radius will be either 24" or 34" instead of the 18"-24" of On30. None of Bachmann's On30 items are appropriate for the Wild West.
HO is the most popular scale and has the most offerings. If the scenery is important to you (and finding moderately priced (scale) figures...it is the scale).
There are a few approaches to the Wild West in HO:
-Bachmann's old time 4-4-0s. They are famous for not running all that great...but they are ok...they also cost about 1/10th of what you'd pay for an O-scale 4-4-0...note that this is NOT the HO Spectrum 4-4-0 which is a 1920s locomotive. They are appropriate for the late 1960s-1880s. Bachmanns old time rolling stock is adequate.
-IHC's old time 4-4-0 and the old Rivarossi 4-4-0s...again, IHC offers a modern 4-4-0 which is different from the old time 4-4-0. They are accurate for the 1870s-1880s. The problem with them is that they aren't HO scale...they are OO scale (1:76 vs. 1:87.1). They at least used to look nicer and run better than the B-man 4-4-0s.
-MDC/Roundhouse by Athearn are post 1900 locomotives packaged as 1880s locomotives (same thing with their freight cars). MDC Roundhouse/Athearn is the source for wild west passenger cars with their 50' overton cars (rivarossi offered them as well).
-Brass engines, while older...more expensive...and possibly needing repowered (which can be hired out)...are the nicest power...and would possibly be cheaper than an O-scale 4-4-0.
-Mantua's old General was a pre-civil war engine as it looked in the 1870s. Mantua has adequate freight cars.
The Wild West was really the 1870s. The 1st Transcontinental railroad had just been finished and many feeder routes were being established/Indian treaties being signed. Also important was the begining of the US narrow gauge movement. While std gauge roads were largely devoted to 4-4-0s, narrow gauge roads quickly added more variety...2-6-0s, 2-8-0s, 4-6-0s. Trains were relatively short, and the small towns have mostly primative buildings. The first of the ng roads was the D&RG (1871) south out of Denver...it had a mix of 4-wheel and 8-wheel cars pulled by 2-4-0s and 2-6-0s. Within a decade, it operated many 2-8-0s as well as 4-4-0s, 2-6-0s, 4-6-0s, and 0-6-0s...and 1 of the only 2 double Fairlies ever to polish American high iron. An interesting event was the Rio Grande war which involved hired gunmen.
The Rio Grande was far from the only road. There were quite a few. The other most revered Colorado ng roads from before 1890 were: the South Park (1874), the Colorado Central (1872), the Rio Grande Southern (1889), and the Silverton (1887...not the same as the Rio Grande's branch which is now know as the Silverton). The two best loved Std Gauge roads of the Wild West were the Virginia & Truckee...and the Colorado Midland (which was actually from the later 1880s).
If you are interested in narrow gauge, look up:
-White Pass & Yukon
-The Denver & Rio Grande (became the D&RGW in 1920...the Durango & Silverton and Cumbras & Toltec are tourist lines on the old D&RG...with 1920s locomotives)
-The Denver South Park & Pacific (became Colorado & Southern in 1898)
-The North Pacific Coast
-The South Pacific Coast
Here is the Eureka & Pallisades #4 on the C&T...http://www.youtube.com/watch?v=L9d2WhZ7kHo
MMI/Precision scale is coming out with this engine in On30 and On3.
Look into Large Scale if you have the space. It is more economical (imo) than O-scale...and more Wild West products are available. Spectrum's F scale 4-4-0 is the same as the MMI 4-4-0s....both in price $400...and in prototype.
I'd suggest picking up a Bachmann HO trainset...they are cheap. If it is too small, look into O and F. One of the scales is probably right for you...and its cheaper to start with HO than the others.