Lionel starter sets and Atlas O scale sets are very nice values--at least SOME are. You really must get a catalog and look at all that is available if you want a really GOOD starter set. And you get what you pay for --$400 starter sets from Lionel are excellent and have many features with digital control, while the $200 area has toy stuff with limited action. MTH starter sets IMHO aren't as big a value as the Lionel and even look more toy-like--at least the steam offerings. For example, the Lionel Polar Express Set is gorgeous, easy to setup and has whistle sounds. You get Fast tracks, a nice transformer--and you get the option later to add complete sounds /steam effects for the engine because Lionel sells a Polar express accessories package--a tender with electronics, more cars,etc. You can upgrade the set so it isn't just going around the tree and then forgotten!
The Lionel Fast tracks are nice, and the MTH Real Trax are nice too--both have pluses and minuses (Lionel track is cheaper$ and MTH is pricey but stronger). Also, if you like to shop in person, your local shop may not sell both track systems, which for some might be a deciding factor.
I love the Lionel TMCC because its price and functionality and compatibility. But there are great things to be said about MTH DCS too. Most of the O gauge stuff out now is light-years better and gives you more choices than ever before so really--(for the most part) its all good stuff. Some people are happier controlling toys, some people are happier with less features on a detailed model; you must decide where you fit in