Design a JEE / Jena TDB / MVC web application - best practice

Hello,

I am creating a web semantic application, were persistant data is all
stored in triple store (TDB).
I mean, users(foaf), parameters etc.
This application is an e-learning application, with exercices
proposition based on reasonners.

I want to use a classic MVC design pattern (jsp / servlets / javabeans).
So a javabeans user, teacher, and student with heritage.
My naïve aproach would be to initialise jbeans from TDB, using jena
model getInstance etc. Or Sparql Querys with ARQ
and my setters will do both :
- set jbeans propertys
- modify triple (dataset jena setProperty ; dataset commit , OR SPARQL querys)

Example bad pseudo-code:
Class student extends user {
private Integer age;
private Ressource student = myModel.getIndividual(
"http://www.myexample.com/mySchema#student1" );

public void setAge(Integer age){
this.age = age; //modify jbeans property
student.setPropertyValue("http://www.myexample.com/mySchema#age","1^^integer")
# modify TDB valyue

1) Are there good practice or some DAO to design ?
2) In term of performances, is it better to use ARQ (sparql) OR jena
ontologie API ?
(3) is it a good choice to store all in a triple store ? (versus
traditional relational databases) )

Thanks a lot

Nicolas PARIS