Trying the two on would be the best thing you could do. Myself and others have had problems with the Platinum sliding down (even though I adjusted mine all the way in). Switching to the Gold solved this for me and others. The nice thing about the Gold is that it is sized by coat size so I think it fits smaller frames better.
However, I also know some people who, like Brian, swear by the Platinum and prefer it over the Gold. Some people also find that the Gold does not extend far enough down and leaves the bottom of their ribs exposed.
Although it may not be easy for you to do, trying both on and seeing which fits your body better would be ideal. Could you maybe order the Gold from ump-attire also and return it if you don't like the fit.
Both protectors take time to mold to your body though and fit better after they've been worn for a few games. As far as the bulkiness of the Platinum's shoulders, mine tended to fit better and appear less bulky after being worn in a few games. I ended up switching to the Gold because the Platinum wouldn't stay in place and was sliding down leaving my collarbone area exposed.