Same rocket but the Big Bertha has a longer Body Tube (BT). If you have access to extra tube, as many of us do, the baby Bertha kits are much cheaper. Buy that and swap out the BT. if not, I suppose it's your call. Both are fun.
Do both. I have 2 Babys and 2 Bigs. I had 3 Bigs, but one was sacrificed to a rocketus eatemupus. I also have a Tiny (BT5) and a Mini (BT20). Next is a Midi (BT50).
I never liked the Berthas, Big, Mini, or Baby. I think it's because of the blunt nosecone and fin shape. But if YOU like them, build, build, build (and then fly, fly, fly)!
Get both and put a 24mm motor mount in the Big Bertha to make it more exciting. I have 2 of each plus my grandson has a Big Betty.
Here are a couple of pictures of my Baby Bertha's.
The yellow one has the red ones nosecone, because the one that came with the yellow would not fit. I sanded it until there wasn't much left, so into the trash. The red one has a nosecone from a Loadstar 2, that I had lying around. I think it really gives it a retro look, maybe round the fins off at the shoulder, and curve the inner trailing edge.