My thought is: Whatever makes you go faster. Don't intentionally use a harder gear to build leg strength- that'll come anyway, and you're liable to start having knee problems if you try to work things that way. You don't necessarily need to work on "speeding up" pedal speed, you may have plenty of that already, but you need to keep it high enough for best all-around performance.
FYI, using tougher gear is "mashing". Speeding up pedal revolution is "spinning". Rate of pedal revolutions is "cadence".
I've read where one of the racers was asked, "Is it better to spin a small gear or mash a big gear?" and said, "Well, in racing, you pretty much have to spin a big gear..."
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"be careful this rando stuff is addictive and dan's the 'pusher'."