As far as I'm aware, no, there isn't a temperament difference. The only thing to consider is if you're breeding them; that tends to cause a bad mood in both sexes... The female because she's carrying babies, and the male because he wants to MAKE babies. lol.
If your girl friend is concerned about a big snake, get a male. All the male boa constrictors stay smaller than their female counterparts. Also, look for a dwarf locality of BCI... Hogg Isle, Nicaraguan, and the Central American locales tend to stay 5 feet and under. Basically a boa in a small package! Nicaraguans stay especially small... I've seen fully grown males the size of a small female Ball python. Hogg Isles seem to be on the large side of the dwarfs, reaching 5'... I've even heard of some 7' Hogg Isle females (though I couldn't honestly tell you whether they were certifiably pure Hoggs or if they were HoggxColombian crosses).
From what I understand, there is a little bit of variation of temperament among the localities. The dwarfs tend toward being flightier and nippier, where the Colombians are more docile... Though that shouldn't be an issue if you pick a tame specimen and work with it on a regular basis.
There are some good boa constrictor forums around, you just have to look for them. I think Redtailboa.net is supposed to be pretty good? You'll definitely get better info from the serious and experienced boa keepers on a good forum than anywhere else. Even what I can tell you here is all second-hand information that I haven't double-checked on since the last time I owned a boa.
Good luck!