I'm 5'6 and curvy, so I think the PST looks small on me. Although I recently tried on the GST, and I even thought that was a bit small for me! I think it's all relative to what your needs are. I don't think the PST is that small that it would look odd or anything like that, but if it really comes down to your own preference. For example, I'd much prefer the GST because I like the look of bigger bags, and tend to carry a lot of stuff.
It's all a matter of preference....I am 5'4" and then wear high heels every day..and I have the PST and love it...I have many big bags that I love so I know the GST would work too...but why do I not have one? It's too boxy for me...I'm not a fan of overly-structured bags...But that's just me...I see pics of GST's & PST's on pfr's here and the ladies all look phenomenal..so it's just up to you and what your needs are..Good luck!!!