try look at pros & cons on both options:

new myvi

pro
look good, feel good
less initial maintenance cost
major part cover under warranty
monies from old myvi can be use for deposit i.e lower starting cost
can opt for better class car..i.e city or vios or saga or any MPV or SUV

cons.
monthly payment
higher insurance premium



old myvi

pro
no monthly commitment i.e more cash in hand to be used for repair and maintenance

cons.
Lump sump cost for repair will need some immediate cash
stuck with old car (hey it's myvi people dont care about it's age so long it can be driven)

The conclusion
it's depend on your financial condition and your own objectives
if only to move from A to B then just stick to old and upgrade when the situation is needed. i.e having a kid and needed a bigger car etc etc..
if u still young save the cash and plan for future..future is not just 5-years ahead with fancy car but beyond that