G is the larger railroad set. However,the O size is usually a bit more scale and realistic and more expensive normally. I bought a G size polar express for my 6 year old grandson, because it was less expensive and less to break on it. but the G is a lot larger. Still very cool tho.
I bought the G train last Christmas for my girls. Gears came apart inside the train after 2 hours of use. Maybe 1 hour. I had to explain to my kids that I needed to send it back to Santa to fix the train. That took some of the magic out of Christmas for my little ones. I'd just spend another $50 on an electric set. Don't get the battery operated garbage. I guess my question to Lionel is how many decades does a toy train company have to be in business to build a train that works? Toy technology has well surpassed model trains over the past 25 years, you'd think they would want to make something that lasts for those few nostalgic parents who would like to see something like this make an impact on little imaginations.