Oh, I'm not jumping on it for a while...but the combination is tempting. The logging railroad also seems interesting. What got me started in thinking about broad gauge is that my laptop's desktop currently is a photo in Cornwall, UK...which was the western end of the GWR.
Art Decko, I think nachoman has hit on a lot of it...I'll try to add my 2 cents...
First, as nachoman said, diesels are cheaper than steam not only due to the fact that diesels were far more standardized, but also due to the lower number of details/moving parts. The standardization gets even worse in the 1900s and pre-1870s. The wheels on a steam locomotive models are commonly more expensive than the motors. Further, model diesels frequently have better pulling power...which means that 4 FTs can pull a very reasonable scale length train while my 2-8-2 can barely pull what one of the diesels could handle (in reality...the 2-8-2 ought to equal two of those diesels).
If you don't really have a preference...a common thing for people whom haven't experienced mainline steam...diesels are a logical choice as they are cheaper. (that being said, I know a guy with a 45x60' basement filled with a diesel empire...and he's experienced mainline steam plenty of times). Most model railroaders today don't seem to be as interested in the building of their layouts, but rather in the operating. That fits well into the mass produced diesel market. It also adds to the aspect of hunting down current prototypes/paint/operations...where I do the same thing through books.
Further, there are billions of pictures of modern steam/early diesels...as well as many surviving examples...but few of 19th century...or especially pre-depression railroads. People tend to model what they've experienced or seen pictures of...so the scarcity of survivors and lack of photos really hurt early modelers. It really shows when you see how there are hundreds of models of the D&RGW 2-8-2s and YELLOW Rio Grande passenger cars...even though their early class 56/60 2-8-0s were the most widely used narrow gauge engines and the passenger cars were GREEN for almost the entire existence of the D&RGW. Making it into the 1950s with steam did a tremendous amount to promote your railroad to modelers.
I love steam. It's wonderful. Diesel is a bad word in my house...but I can still appreciate the work the diesel guys do...I just don't find their engines...or modern freight cars...to be interesting...I'm sure most of them feel the same way about my 26' wooden freight cars