water fall model testing is performed  under low priority 
activity.If any defect  you  can identify the risk,time and 
cost of the s/w is very high.un realistic testing and 
inadiquatetesting perform.

v-model testing is performed corresponding parallel testing 
activity.If any defect you can identify the earlier stage 
so risk,cost and time very low.The expected s/w product 
deliverd.