I have no idea what the quality or service is like nowadays.
DF used to commute from Luton to London Bridge daily and has complained often about the 'standard' of service First Capital Connect offers. Granted, if Dar were doing a commute to/from another town to Luton (missing the into-London mess), it would probably be slightly different. However, for living only 1 or 2 stops away, I can't imagine that the service will be AS relevant as a longer commute...(in other words, don't fret too much about it).
First Capital Connect & Midland Mainline (fast services) are the 2 lines that run through Luton Rail Station. Luton also has Luton Airport Parkway as a station option, but there are less trainline options.
Like other people have said, you need to consider ALL transport options: bus AND train. Both are perfectly viable and important for decision-making!