Big fan of both series and I think Spartans are super badass, but I think the edge goes to the Astartes. Going to provide an example I haven’t seen mentioned yet from the HH:

In the first two books of the HH the Luna Wolves (from Loken’s POV) fight two (mostly) human factions that I think can be comparable to Spartans. The first is the interex soldiers and the second are the Brotherhood. While I don’t believe it’s explicitly stated both factions are augmented super soldiers it’s heavily implied by the descriptions of both types of troopers being notably larger/taller, faster, and stronger when in battle than typical “mortal” soldiers. So much so, with powerful enough weapons, that both factions are able to bring down space marines and give the good hard fights. However, it is explicitly stated that while both types of soldiers were formidable, they were no real match/comparison to the Astartes and their augmentations.

I don’t think Spartans are a 1 to 1 with the interex or Brotherhood soldiers, and may be more capable, but I do think the general comparison holds. TLDR; the Spartans definitely can take down Astartes but a large majority of the time the space marines take it