Besides the fact that the 300 ER (Extended Range, so obviously it could hit a greater range of about 1800 nm more than the 300 at 7.4k nm), the "visible" differences include a longer wingspan of about 12ft longer and 2 in. taller. However, I said "visible" as it is only comparable if you park a 300 and 300 ER side by side. A more noticable difference would be the ER models also has raked wing-tips. Internally, the 300ER has a slightly higher speed limits for flaps and overall heavier weight. Also the ER has a slightly bigger tank capacity, and not forgetting a Tail Strike Protection whereby the elevators would be reduced automatically if the system detects that the plane is about to get a tail strike. Hope this helps.
Source:
http://www.slideshare.net/otton6081/...-erdifferences