Like all clothing, one manufacturer's sizes are not consistent with another. You need to check the size charts applicable to each manufacturer after you have used a measuring tape.
I don't think there really is a definitive answer, sorry.
The best thing to do is go to a show or large tack shop and try a load on. It depends on how fitted you want it and what sort of shirt you think you might put under it. To an extent what discipline you want it for, whether you need to fit a BP over it - or even under it. The only way to know for sure it so go and try! (Bit like normal clothes really...)