Let me first state my experience. I have my own shih tzu who is about 9 months, and I know about 8-9 shih tzus/Bichon from friends and family. Can you tell we love this breed?
The Shih Tzu temperament is very very very. and I mean VERY friendly, lol
As long as they where not treated badly early in life they love people, dogs, inaminent objects :lol-sign:
Both of these breeds can have a tendency to bark to much and so a lot of training needs to be done on preventing that.
Grooming with a long coat needs to be done every day, with a shorter coat every two days. Even with all that grooming I still get mats on the tail, so watch out for that. The grooming costs can also really add up because usually they need to be groomed every 1 1/2- 2 months.
Depending on the dog the energy level can vary. Charlie has a very low energy level most of the day, and then runs around crazy at night. The female shih tzus I have met seem to have a high energy level.
House training is a *****, yea, not good... so I hope for your sake she is house trained already, though I have heard Bichons have an easier time with this, I guess because they get to be bigger that helps.
Good luck!