@Charlie MacPherson well said. A lot of awesome points. The big brokerages do offer great support, but your fees will be high. KW has this great camaraderie and a culture that a lot of people love. However, be very careful with KW. The numbers favor the people who cap, and the little guy (selling under $4 million/year) ends up picking up the tab. It is a great brokerage and system, but it is the most "pyramid" style business in the real estate business. I am not knocking KW. They are good at a lot of things and if you need motivation and you want others to push you (because you lack motivation or you just like doing cartwheels in a room of a hundred people at the same time), then KW is for you. The other brokerage, Coldwell you mentioned, has good training, lower splits, and much lower fees (and they don't charge you per page on the copier)
You do NOT need to affiliate with a broker until you are ready to get licensed, so you have plenty of time. I would make one recommendation and that is to look up some smaller brokerages to see what other agents (or even their customers) have to say about them. Sometimes you will find the smaller brokerages who charge zero fees, which can save you a tremendous amount of money. That said, the smaller firms are just that, smaller, with less awards banquets, and all the hoopla that can attract new agents (shiny object!!) Coldwell will pay you a 50-60% split, while KW says they are giving you a 70%. Keep in mind, they charge you 6% on top of that, so technically it's only a 64% split. And while there IS free training sessions at KW, a lot of the good stuff costs money and is why their business model is so successful.
Good luck with whatever you decide and I hope my points gave you some more to think about and to look into more options than simply those two.