The foremost argument used by proponents of Nasbeh is the 9th century border conflict. Fortifying Nebi Samwil instead of Nasbeh would still lead the main road safe for Baasha to re-fortify Ramah/er-Ram. Nebi Samwil also has no Iron Age remains before the 8th century, while Tell Nasbeh was always an important Iron Age site. The Seal of Jaazaniah (2 Kings 25:23, Jeremiah 40:8) found at Nasbeh also supports its identity as Mizpah. Jeremiah 41:5 also supports placing Mispah on the main road.
The foremost arguments used for Nebi Samwil, identified as Mizpah fifty years before Nasbeh, are the arguments from the Joshua list and the fact Ishmael, planning to cross into Ammon, went through Gibeon on his route (Jeremiah 41:10). These arguments are hardly decisive. Ishmael could simply have taken a less crowded and conspicuous route to Ammon. Joshua 18:25-6 goes from Gibeon to Chapirah East and West in an alternating pattern, if Mizpah is put at Nasbeh (the pattern is broken if Samwil is Mizpah).
In short, Tell en-Nasbeh has clear support of being Mizpah in Benjamin both topographically, geographically, and archeologically.