A Minion attack and a Militia attack are roughly equal on average, I suspect. One leaves you with 4 average cards; the other leaves you with 3 good cards. You'd rather be Minioned if you're playing the kind of uniform deck Envoy is good with; you'd rather be Militiaed if you've got everything from power cards to curses.
The big difference is that the first version of the card only did half a Militia, and the second version does a full Minion. I'd say $4 for the first, for the excellent reasons others have already posted. This may be one of the few cases where the difference between $3 and $4 is significant after all, as Tables and Davio have said. (Throne Room is another example where that difference is significant, and for the same reason.)
The second version is more powerful, but is it worth $5, where it competes with the actual Minion card? I'd say it falls far short of a real Minion, and yet I can't see pricing it any less. It's a pretty powerful attack to put on a cantrip, and the fact that you can throw one into virtually any kind of deck to significantly slow down your opponents is pretty rough.