国内申請のガイドの「Important Information: For Sponsors and Spouses or Common-law partners」(
http://www.cic.gc.ca/english/applications/guides/5289E.html#wp1048165)に、国内申請の不利な点が書いてありますよ。
プロセスタイムについては、次のように記載されています。
Family Class redesign, which is aimed at faster processing of spouse or common-law partner applications, only applies to applicants outside Canada. Processing times for spouses or common-law partners in Canada are generally longer.
You or your spouse or your common-law partner:
Can apply to a visa office outside Canada to take advantage of the Family Class redesign processing standard