The key is not smoking too much. Heavier indica strains will give you couch lock, detract you from your workout and make you slothful. Heady sativas will give you a zippy high, your mind will be racing, the mind-muscle connection will be unlike anything else (aside from maybe speed and nasty shit), you will probably even lift heavier because of the stronger mind-muscle connection.
I am fortunate to live in BC where I can get tons of strains. I have friends who get name brand strains from compassion clubs so I know exactly what to expect. I've found that heady sativas are the best. The heavier indica strains are okay too, and it really helps to loosen you up. But if there's a heavier ratio of cannabinoids to THC then you'll be tired and slothful. So many people think that smoking weed makes you too lazy to lift, but that's because they're smoking shitty weed or just too much. You only need one or two puffs. I take a puff before my workouts and always lift with insane intensity. When I don't smoke is when I often feel less motivated, but I do muster up the energy and motivation to have a good workout.