Hi Lawnchoker,
Welcome to the gauge first of all. Second, you have chosen a great scale. There are good reasons to chose either MTH or Lionel. The biggest difference between the two is compatibility of the control and sound systems. Just about every company that offers engines with "control" uses the TMCC system. The exception being MTH, who has their own DCS system. I think DCS has more features but it may not work with the next version that comes out. Whereas TMCC works with all command controlled engines made. Track is the other thing to consider. If you go with MTH, they have their RailTrax line that comes in starter sets. This has molded in roadbed and snaps together. Once again, this is great if you only use their stuff. But if want to add Lionel accessories such as the milkcar, culvert loaders/unloaders, cattle/horse corrals etc. that use track as part of the setup, RealTrax won't work. Lionel starter sets come with O27 tinplate track. This is nice to start with and some people build layouts with it, but not all engines will run on it. Some engines require O gauge track, this is a little taller than O27. Most people upgrade their track after getting started, so don't worry too much about this. It all is basically your preference, I would start with Lionel and go from there. All the rolling stock is compatible with each other so there is no problems there. Enjoy and if you have more questions just ask. Steve