I'd translate them as follows:
分かるわけがないでしょう。 Wakaru wake ga nai deshō.
"There's no way (he) can understand this." or "(He) surely doesn't understand this."
分かるはずがないでしょう。 Wakaru hazu ga nai deshō.
"I don't think (he) understands this."
はず is more of a personal inkling, while わけ is more of an objective/direct/strong statement. Depending on how it's meant わけ can be used insultingly:
出来るわけないだろう。 Dekiru wake nai darō.
"(You) can't (are unable to) do that!" or "Are (you) crazy to think (you) can do that?"
出来るはずはないけど… Dekiru hazu wa nai kedo…
"(I) don't think (you) can do it, but... (maybe I'm wrong about that)"
More, negated examples:
出来ないわけがないでしょう。 Dekinai wake ga nai deshō.
"It's not impossible, right?" or "It should be possible, right?"
出来ないはずはないだが、今は難しいです。 Dekinai hazu wa nai da ga, ima wa muzukashii desu.
"I don't think it's impossible, but it's very difficult right now."