While the Kissimmee station is technically closer to WDW, keep in mind that it is not a full service station like Orlando, hence many folks typically prefer to use the Orlando station.. The Orlando station has more amenities, concessions, transportation services, car rental pickup, a large indoor waiting hall, etc.. These services are either not available or very limited at Kissimmee station. Just something to keep in mind, especially if there are delays or inclement weather.
Kissimmee does have an indoor waiting room, although it's small and seating is very limited. IMHO, it retains the charm of a small town early 20th century rail station. Orlando is large and architecturally impressive with plenty of seating. There are rest rooms at both stations.
Both stations are one-person operations, offering checked baggage and ticketing.
There is no car rental desk at either station. As per PP, car rental via pickup or advance arrangement might be available at either, if the train arrives during normal business hours.
There is no permanent food service at Orlando. I've been at the station 3 times while a train was at the platform; one time there was a hot dog and sandwich cart set up, the other 2 times there was nothing. OTOH, Willy's Wieners is just around the corner from Kissimmee station.

There are several other restaurants within a couple of blocks, as the station is located in Kissimmee's historic downtown area.
Kissimmee station is located next to the city's main bus station, where there is
a bus that runs to TTC at WDW. At Orlando, there's probably a bus that runs down Orange Ave. which is about a quarter mile from the station, but it does not go to WDW.
Both stations have small parking lots. Orlando's is larger than Kissimmee's. But Kissimmee also has a large, free public garage just across the tracks.
Both stations have have free-standing shelters over parts of the platform, and benches on the platform.