Tough call! Both combos are very handsome but [name_m]Callum[/name_m] wins for me. It just has such a handsome yet gentle quality to it. [name_m]Declan[/name_m] sounds a bit harsher to me.
I prefer [name_m]Declan[/name_m] in general (love one of its purported meanings, “full of goodness”!), but I like the sound of [name_m]Callum[/name_m] better with your surname. For some reason I just found [name_m]Declan[/name_m] Funkhouser to be really hard to say, and I’m not sure why! Maybe it’s that the "C"s are closer together than with [name_m]Callum[/name_m] Funkhouser? Either way, they’re lovely names!