Its generally a good idea to go for the shortest term possible in your circumstances (as you pay less interest). However, if you think the 20 yr term will stretch you too much, you could take out the 25 yr term and reduce it in the future.
For instance, if you take the original mortgage over 25 yrs and you decide to decrease the term, you can do so. If you then find it the higher repayments too much, you can increase it back to the original term. It is probably easier to do this than trying to extend the original term.