There's no easy answer; depends on the plastic and conditions. If it's a full bodied 5" shad-type swimbait, the 3/4 oz is likely a perfect match. If the 5" bait is slender or has a thin tail such as on a single tail grub, a 1/2 oz or even a 3/8 oz may work well. Also, water depth, current and desired rate-of-fall affect your choice of head weight. Without seeing the bait you're referring to, I'll say get the 3/4 oz and 1/2 oz, start with the examples I used above and change as needed.
Also, notice the people around you who're getting bit and mimic what they're using/doing. If using plastics with jig heads, see what size head and plastic they're tossing, if they're counting down after the lure hits the water and they're rate and style of retrieve. Nothing works like proven success; don't be shy about aping the guy who's catching the most fish.