Hi Itay,
Salt water aquarium is a great idea and opens fantastic options (colours, sizes.....) but please remember that:
1.  It is (as you mentioned) much   
     more expensive).
2.  Much more sensitive to all the
     factors (temp, salinity, .....).
3.  Much more sensitive to deseas.
4.  Needs much more care and
     attention.  That means you can't
     leave it for long time and ask a
     friend to feed them......
However, if you are ready to all that it's a great idea.
Best regards,