I have the Small Chocolate Antony and I tried a Seth on (mainly because it had a detatchable strap that I could use on my Roxy and I wanted an Oak Messenger too and thought I would get a different one rather than a second Antony) however, I thought that the Seth was too big for me.
I'm 5ft 4in and I felt that the Seth was too long really. I prefer something wider rather than tall - I just thought it made me look shorter.
I think the Small Antony is perfect because when I use mine I only carry my Purse, Phone, Lipbalm, Handwash and Keys. Its the perfect size for that lot. If you want to carry more it might be a little bit small.
Take a look at the What's IN your bag thread because there are a few photos on there of what will fit into an Antony, it might help with your decision.