The argument against either Biden testifying is straightforward: Trump is being impeached for using the power of his office to smear his opponent, so he shouldn’t be allowed to use his impeachment trial to further smear him. Having said that, Republicans are clearly going to do that regardless (my senator, Rick Scott, is currently running TV ads in Iowa repeating those smears, even though he’s not running for anything) so maybe the best option for Biden is to take it on directly and spend a couple days spitting fire at Trump.