Calculate what you want your return pump flow rate will be.
Then look up the "max siphon flow rate".
http://www.monsterfishkeepers.com/forums/threads/pipe-sizing-charts-and-flow-rates.205645/
If 3/4" will support the max siphon flow rate you want, use that for your primary full siphon drain.
If not, then you will need to use your 1.5" as your primary drain and your 3/4" as your backup.
These configurations will get you the "quiet" overflow. Similar to Bean animal, but slightly not quite as safe.
It is safe enough if you are listening to your drain pipes to make sure they are not clogged.
Example 1
if you expect to do 300GPH, then you use 3/4" as primary drain pipe and put a gate valve to make sure it is at the siphon drain rate to just barely match your return pump rate.
Then your 1.5" will be your emergency/backup drain that will either run dry (or typically people run it at a trickle flow). You control the trickle with the gate valve on the 3/4" primary drain.
Example 2
if you expect to do more than 300GPH (like closer to 1.5" siphon flow of 1300 GPH, then you use 1.5" pipe as primary drain pipe and put a gate valve to make sure it is at the siphon drain rate to just barely match your return pump rate.
Then your 3/4" will be your emergency/backup drain that will either run dry (or typically people run it at a trickle flow). You control the trickle with the gate valve on the 1.5" primary drain.
NOTE: for both drain pipes, the drain pipe going into the sump should be submerged just about 1" below your typical sump water level.
This means the drain chamber in your sump must be a constant water level (typically your skimmer chamber works).
Any deeper than 1" deep will make it hard for air to exit the pipe easily. Too short and you may get more gurgling effects if water level changes a bit and can break the siphon effect.