Well IMO, they are putting the 3.73 in there for gas mileage, you will however notice a difference in performance by upgrading to the 4.10. There are a few websites where you can calculate the difference in engine RPM's etc. for your various gear ratios and tire sizes. That would help you determine. I'm guessing you'd see about at most 100-200 rpm difference at various speeds. To me, the biggest factor would be the gas mileage, if you want maximum, then stay put, if you're doing A LOT of highway cruising etc. If you're a city slicker and don't care so much about MPG make the upgrade.
2007 Liquid Gray Focus ST Gas saver
2003 FX4 Sonic Blue Sold!!! Gas guzzler best tank ever 19.5 MPG