I highly recommend speed chess (5 mins), as it allows for many more repetitions. You'll learn faster what works / doesn't. Intersperse every once in a while with a slow game to rethink your fundamentals and strategy. Also, speed chess is much more forgiving. 1 mistake is not the end of the game, as time is the great equalizer. Whereas in slow chess, going down a pawn unintentionally is often a loss.
While great fun, I don't think playing blitz is beneficial in the long run. It encourages plenty of bad habits: guessing (instead of calculating), "gambling" (making bold but refutable moves in hope that opponent misses the correct answer), over-relying on simple tricks like common opening traps which wouldn't work and would get you into an inferior position in a serious game, etc. etc. Good blitz players are good ch…
>Good blitz players are good chess players who happen to put their skills to a use by playing blitz, but they haven't developed these skills just by playing blitz
I'm a class A player, and I've probably played < 20 slow chess games over the past 15 years.