Berdych does have a better serve
He has a better first serve definitely. Del Potro gets a higher % of first serves in and his second serve is far better/less attackable. I'll definitely take JMDP's more reliable serve over Berdych, who has a world class 1st serve but poor % and a weak second serve.
Anyway, the only reason this is a contest is Del Potro's constant injuries , he's definitely clearly the better player of the two.