The WRT54GX may be a better choice if it is pre-determined the WRT54G will not have sufficient range, given that the GX's price is more than double that of the G's.

However, the WRT54GX uses MIMO to enhance speed and range performance. MIMO is a non-standard performance-enhancement technology that doesn't solve WLAN problems caused by an already over-crowded 2.4GHz band. More importantly, it has minimal chances of being compatible with 802.11n (the next standard) and may therefore limit flexibility when upgrading or expanding.

That said, it really depends on how the network will be used and expanded in the future. If the objective is simply to have optimal range and speed without consideration for the future, then the GX may yet turn out to be the most logical choice. (Caveat: client side must use GX-based receivers for optimal range and speed. That means additional costs.)