I think if you had a "both" option, it would blow the others away. I would say 75% of the guys I know run both in some way. The majority (and I think the posts here confirm this) tend to model the 30's-50's, often specifically so they have both options - that would be my tactic. Some model later periods, but 'just happen' to have a nostalgia-trip steam train that runs in their area or the 'last remaning' shay creaking along on their logging line. I even know a fellow who, with the judicious removal of a couple buildings, runs a dual-era pike that can shift from 80's to 50's in 10 minutes. I'd actually like to see a poll asking for people who run ONLY steam or ONLY diesel versus both.