"Do both systems use 18v DC. Not to sure about running two different control systems just on a carpet central, the MTH looks more complex. Would also need transition track pieces to go from fastrack to realtrax. Then would also have to get a hand held controller, cant use my wifes iphone all the time, I am old school carry a flip phone. "
There's no compatibility problem voltage-wise, or command signal-wise between the MTH and Lionel systems. MTH's system is called DCS and the locomotive system is called Protosounds 3 (PS3). They operate in conventional mode (vary the voltage) or with a smart device (iOS or Android) or with the DCS system (about $300). Lionel actually has three systems, all of which work together seamlessly. The inexpensive LC set locos and LC+ separate sale locos use a command system that is simple but reliable. You use the remote that comes with every set or loco. Or the universal remote that controls locos with Bluetooth or LC/LC+. Newer Lionel LC and LC+ locos also have Bluetooth. The upscale Lionel system is called Legacy (about $350) and new Legacy locos can be controlled by Legacy or Bluetooth, or in conventional mode. LC locos do not function in conventional mode, but LC+ do. This sounds more complex than it is, and is Lionel's approach to providing command control, multiple remotes, etc. inexpensively.
You will need a smart device (iOS or Android phone or tablet) to operate the MTH set. There is no other method of control included (unlike LC or LC+, where every set or loco comes with a dedicated remote). Not a big deal for most folks, but spending $400 for a set and having to provide your own control device(s) is not everyone's cup of tea.
I would stick with Fastrack, which comes in every Lionel set, and is generally accepted to be a higher quality product, and you have some of it already.
Another thing to consider is value. The MTH set's value is in the quality of the locomotives and rolling stock. MTH sets have higher quality locomotives and rolling stock, in general, and thus are priced accordingly, $100-200 higher than the cheapest Lionel sets in most cases. You absolutely need a smart device for the new MTH sets, as without one you cannot access the set loco in command mode and benefit from the PS3 system. There is no method of control, remote or conventional, included in an MTH set these days. If finances are a major consideration, that additional $100-200 can be spent on additional track, an operating accessory or two, or additional rolling stock.
For more advanced hobbyists, having both the DCS/PS3 system from MTH and the Lionel system (Legacy) is worth the total $700-800 investment. For many, myself included, the inexpensive command system using LC and LC+ locos that come with the included remotes, or purchasing the $50 universal remote to control 3 locos, and/or Bluetooth provides more than enough function and good value.
There's no rush to make a decision, I'd guess.