I will have to say I am a big fan of the '04 and up Sportsters. They are extemely low maintenance (hydraulic lifters, so no valve shim check). The Sportster Custom sits very low and comes with a 4.5 gallon tank, and looks really cool, and is in the same price range as the Speedmaster.
Triumph has always been in my blood and I like the Speedmaster as well, a very sporty look, and they sound really good with aftermarket pipes. Like our Rockets, not everyone has a Speedmaster, so it may get her a little more attention as well.
If she's like my wife, she says she's not worried about the attention, but... when she pulls up on her Trophy at the local bike night and people start gathering around asking her questions, she sure seems to enjoy it..
The Sportster Custom is closer to the Speedmaster in specs and price. The Sportster low is a very, very basic bike and does not sit that much lower than the Custom. Also, you get stiffer shocks with the Low to help make up for the lack of suspension travel.
Basically, you can't go wrong with either one. I would let her decide and go from there. My wife test rode the Honda ST1300 and Yamaha FJ1300, and she is the one who decided to get a used Trophy 1200 instead, and she couldn't be happier!