William Bade excavated Tell en-Nasbeh from 1926-1935, & based on his findings, scholars agreed that the site represented Biblical Mizpah (which means "lookout" or "watchtower" in Hebrew).  However, Yitzhak Magen excavated Nebi Samwil from 1992-2003, & challenged Nasbeh's longstanding association with Mizpah (BAR vol. 34 #3, May/June 2008; pp. 36-45, 78-9).

• High view of a key road to Jerusalem
• Many First-Temple, Persian, & Hellenistic artifacts
• Seal of Jaazaniah (2Kings 25:23)
• Close proximity to Geba (assuming it's modern Jaba; a popular association)
• Located north of Ramah (assuming it's modern er-Ram; a popular association), which would defend it from being recaptured by the northern kingdom (1Kings 15:22; 2Chronicles 16:6)
• The names are phonetically similar

Points for Samwil=Mizpah:

• High view of several key roads to Jerusalem
• Some First-Temple, Persian, & Hellenistic artifacts
• Close proximity to Jerusalem (1Maccabees 3:46)
• Close proximity to Gibeon (el-Jib; an undisputed association; Nehemiah 3:7; Jeremiah 41:16)
• Close proximity to Geba (assuming that Geba=Gibeah=Tell el-Ful; a doubtful association), which would defend Jerusalem from a northeastern approach by the northern kingdom (1Kings 15:22; 2Chronicles 16:6)
• Located south of Ramah (assuming it's modern er-Ram; a popular association), which would defend Jerusalem from a central-northern approach by the northern kingdom (1Kings 15:22; 2Chronicles 16:6)
• Located between other cities listed in Joshua 18:25-8
• 12th-century A.D. map associating Samwil's "Mons Gaudii" monastery with the name of Mizpah

"Gibeon, & Ramah, & Beeroth, & Mizpah, & Chephirah, & Mozah, & Rekem, & Irpeel, & Taralah, & Zelah, Eleph, & Jebusi, which is Jerusalem, Gibeath, & Kirjath; fourteen cities with their villages."--Joshua 18:25-8

"Then king Asa made a proclamation throughout all Judah; none was exempted; & they took away the stones of Ramah, & the timber thereof, wherewith Baasha had builded; & king Asa built with them Geba of Benjamin, & Mizpah."--1Kings 15:22

"Then Asa the king took all Judah; & they carried away the stones of Ramah, & the timber thereof, wherewith Baasha was building; & he built therewith Geba & Mizpah."--2Chronicles 16:6

"...there came to Gedaliah to Mizpah, even Ishmael the son of Nethaniah, & Johanan the son of Careah, & Seraiah the son of Tanhumeth the Netophathite, & Jaazaniah the son of a Maachathite, they & their men."--2Kings 25:23

"And next unto them repaired Melatiah the Gibeonite, & Jadon the Meronothite, the men of Gibeon, & of Mizpah..."--Nehemiah 3:7

"Then took Johanan the son of Kareah ... all the remnant of the people whom he had recovered from Ishmael the son of Nethaniah, from Mizpah, after that he had slain [people] whom he had brought again from Gibeon."--Jeremiah 41:16

"Wherefore the Israelites assembled themselves together, & came to Mizpah, over against Jerusalem; for in Mizpah was the place where they prayed aforetime in Israel."--1Maccabees 3:46