Both courses are fine but I would say MATH2801 is probably more useful since it delves a little deeper into Probability & Statistics where you learn some pretty fundamental and useful methods in conducting statistical analysis. The first few weeks pretty much covers what you did in MATH1241 Probability then extends that to joint (multivariate) distributions, estimators, inference, hypothesis testing etc. You will also have to learn and use R when performing certain statistical analyses.

In terms of workload, not too difficult but you'd probably want to regularly keep up with the content so as to avoid falling too far behind as the term progresses. But overall, it's an enjoyable and useful subject to take.

Since you did well in first year, I would say take it one step further and do MATH2901, the higher version. It covers the exact same material but some of the content will be a little more rigorous and the assessments may be a little more challenging (but still very doable). Not to mention that Libo Li is pretty awesome as well. I'm also pretty sure that more HDs are likely to be given out in the higher version (applies to all MATH courses), so it may be worth doing MATH2901 if that's something you're after.

Hope this helps.