rue Beaurepaire is fine-just about a 5 minute walk to Republique.

But I think so far I would chose the one in the 2nd. If it is close to Sentier that will put you near the top of Rue Montorgueil, one of the nicest walking market streets in Paris. You will only be about a 10-15 walk to the Louvre and river. Even though I don't like the Les Halles area, it has been completely remodeled in the past couple of years so it is nicer than it used to be.

The one near Voltaire should be okay too-that puts you just north of Bastille which is nice these days and kind of just east of the Marais so a pretty good location too.

In general all those neighborhoods are fine in terms of safety, etc. In general, most people probably prefer to stay closer to the river when they can versus the neighborhoods limits to the north-but that is a very general statement.

Grand Blvds area is really just north of Sentier-so if the apartments are more or less equivalent then I would definitely choose Sentier for location.

Anyway hope this helps-I lived a few years fairly close to the Sentier area near Etienne Marcel and it is a pretty nice area and I think it would provide the kind of location that visitors would enjoy.