I do not think them the same. For εἰ μὴ supposes already that there is that one of the kind to which the negative generally applies; it is an exception. But ἀλλὰ retains its adversative force as to the whole, but something modifies it in result. Thus in Matt. 11:27, there is one who knows- no one else except;-in chapter 12:4 it was lawful to none else except. In Matt. 17:8 they saw no one, οὐδένα εἰσον εἰ μὴ τὸν. In Mark 9:8, "and suddenly looking round," οὐκέτι οὐδένα εἰδον, ἀλλὰ τὸν Ί. Here the scene had disappeared, but they saw Jesus alone with themselves. So in Matt. 20:23, Mark 10:40, οὐκ ἔστιν ἔμὸν δοῦναι, that is all denied—only modified by ἀλλὰ οις. He does not give places at all as His will, or His patronage, but to those for whom, etc. In Mark 10:18 and Luke 18:19, if not Matthew, we have οὐδεὶς ἀγαθὸς, εἰ μἠ εις ὁ θεος. Naturally, good ones were before His mind: He excludes all but God.