For me, the control system is the thing. I want PS-3 but can't usually afford it. So I buy used PS-2 and used TMCC. The layout is not set up for conventional so I don't buy any conventional or PS-1. Of course anything can be upgraded to PS-3 for $175. So if you can get a loco cheap enough, buying PS-1 or conventional still works. But usually they are not priced to allow for the upgrade to be added and still get your money back if you ever sell it.
I wish MTH would open-source their DCS or Lionel would do the same for legacy. That would really blow the market wide open. But that is sheer fantasy. It will never happen.