The textual variant is between ἡγιασμένοις, sanctified, and ἠγαπημένοις, beloved.
"Beloved" (ἠγαπημένοις): א, A, B, Ψ, 33. Favoured by modern critical texts (NA28/UBS5, Nestle-Aland) and translations (e.g. ESV, NIV, NASB).
"Sanctified" (ἡγιασμένοις): K, L, P, Byzantine majority, and later Greek fathers. Followed by the TR and translations like the KJV/NKJV.
Internal Evidence: "Sanctified" gives a grounding to the use of "saints." "Beloved" aligns with the call to keep yourselves in the love of God. So basically internal evidence does not lean either way. On a broader scale both words are used in other NT epistles.
So basically it comes down to how you evaluate the ms. evidence. Manton defended "sanctified" because he relied on the TR which was based on Byzantine mss. The reading "beloved" lacked ms. support in his time as this was the pre-critical text era. If one holds to the idea that God has preserved His word it follows that the correct reading is the one that was carried through to the reformation and the time of printed editions. If one is open to the idea that readings can be lost and found then the older mss. will carry weight with you since it is assumed that they will give you the earliest reading. On this basis Metzger argues that "Beloved" is virtually certain due to superior manuscript support.