MBLG1001 and 1901 share a large proportion of the course(s) and the differences are that:
in MBLG1901, you get like 13?ish separate lectures from normal MBLG students where you learn about experiments that led to the discovery of proteins and DNA structure and so on. That is not to say that you only learn about theories of experiments, but that is indeed the main theme that is added onto the normal MBLG topics (and to fit them in, luckily some of the (useless?) details in MBLG1001 are removed!) But some 1901-exclusive topics are also theoretical so you get a mix of both more advanced theories and experimental stuff.
And don't worry about those things in 1001 but not in 1901 because you will get separate final exam papers and you will only be examined on things from common and advanced lecture slides.
In regard to exams, I found MBLG1901 exam actually EASIER than MBLG1001 exam (2010). I ofc didn't do both exams but I've asked my friends from 1001 what questions they've got and the gist is that in 1001, they tested you more on difficult-to-remember details, while 1901 replaces some of them by contents from advanced lectures and in 2010, they were one adv theory question (which was more meaningful a question than pathetic details question in 1001) and more on experiments which are mostly give-away questions if you crammed them!
And another thing: only the short answers sections vary between the two exams. I think you still get the same set of MC questions.
In regard to labs, 1901 is the way to go. It's because you actually get some space in labs if you do 1901! 1001 labs are so packed with people and aren't the best lab learning environments. The downfall is that you get to write more reports and they are marked more harshly than normal ones. But there's scaling anyway so I don't think it matters much!
P.S. whoops, forgot to talk about whether it's hard.
It's said to be the most infamous 1st year science course, but the (in)fame really only arises from the amount of content in the course and long labs. Individual concepts/facts are not very hard, but there are lots of them.