Yes it is hard to beat the Lionel F3 in Santa Fe red, silver and yellow. Trouble is I do not have one, just a repainted Marx F3 ABA set. Still pretty good looking.
Charlie
I would prefer the Big Boy or the Triplex and my second choice the F3 Santa Fe red,silver and yellow or the GG1 Pennsylvania
My first post here!
I like anything that moves on rails.
My first train was a loop of track, a steam outline engine, a couple of freight cars and a caboose. I was between 4 and 5 years old (~1945). The engine was a (long departed) steam engine (possibly Marx, but I'm no historian). I do remember that the loop of track was mounted on a piece of plywood. My father and a couple of guys he worked with liberated the plywood from their aircraft company employer right at the end of WW-2.
I grew up appreciating all kinds of propulsion machinery. Lots of memories of steam, diesel and electric motive power.
Just starting on a new (mostly) On30 layout. First motive power will be steam. It will interchange with a standard gauge line using a single Budd Diesel car. Kind of like the NWP and the California Western RR interchange at Willits, CA, except the Cal Western segment will be narrow gauge.
Rereading the question: I would run what I have or what fits to the modeled era!!
Steam. Why? Cause I like it and it's my railroad. 
That part was easy. Now to decide which one steam engine it would be....
To the point - STEAM
Neither!
Under the wire with my B+B-B+B articulated freight motor...
Steam all the way
Steam for both grandson and myself. We both have a special place in our hearts for the Frisco 1630 at the IRM in Union, IL. My first taste of the 1630 was back in the 1970's, courtesy of Phil Hehn... haven't seen him for a long time, not sure if he still volunteers out there or not.
I honestly don't know what I would chose if I could only have/run one engine on my layout. I guess I would have to say a steam engine, since I like the look of the steam engines better running than diesels. As to what steam, I guess first choice would be NKP 765, since I have ridden behind that one, second choice would probably be the N&W J 611, since I have that one, then probably the Big Boy by MTH.
Now, if I had to choose one that my wife would want me to have, it would have to be a 19th Century Steam Engine, since that is the only kind she likes.
