Given the arbitrary manner in which bikes are measured, your current 23 wouldn't necessarily compare to the 520. But given your height, I would guess the 23 to be a better fit.
I had a similar experience. My "Trek" dealer had no 520's in stock. I ordered a 17 because I thought it would fit. Turns out the 17 was too small. (I'm 5'7") I then ordered a 19 which was perfect. The assistant manager was all pissy because he was "stuck" with a low turnover bike. He even tried to convince me it fit. The store owner apologized and had an attitude adjustment meeting with the assistant. The funny thing was, the 17 that he was "stuck" with sold before my 19 was even delivered.
If you're LBS is a "Trek" dealer, they should order the bike for you without requiring your payment, and it should arrive in about a week. This is not a "special order" stuation for them.