Generally speaking, it shouldn't matter. The advantages and disadvantages of taking a 16t tank vs a 32t tank are fairly minimal.
The main advantage of the larger tank is being able to carry an extra 3 jumps worth of fuel without refueling. This is useful if you decide you want to use lots of neutron stars that are far enough apart that you can jump from one to the next.
The main advantage of using the smaller tank is that you get a bit of extra jump range without having to do too much fuel management. You can get the same jump range with the larger tank just by having the tank half full instead of completely full.
The max jump range (completely empty with just enough fuel for a single jump) of you Asp will remain the same regardless of which fuel tank you pick. There is very little difference between the two, and unless you plan on going somewhere where you know you will specifically need to use a 16t or 32t tank, it really comes down to personal preference (I would take the 32t simply because it gives me a little bit more wiggle room when it comes to fuel).