During a RTTY transmission, your transceiver is operating at maximum output continuously. Some rigs have a minimum output level as high as 5 or 10 watts. All transmitters emit some signals outside their intended frequency range. These signals are called spurious emissions - a term that includes all types of signals that are not the fundamental and its desired modulation. Amateurs must be concerned with the level of these spurious emissions, both to ensure that the transmitted signal is in compliance with Part 97, and to ensure that the transmitter doesn't interfere with other services.
Spectral purity is measured using a spectrum analyzer.
Any transceiver advertised in QST must meet the minimum FCC requirements. Earlier you said that convenience was one of the major factors to consider. It's simple, really. Take a good, close look at the radio. You should also consider the owner's manual.