Thebtrick is you are asking what the opinion of one to another, when not everyome knows. I haven't played Halo or read halo books so i don't know.
However i have read near enough EVERYTHING space marine based, am experienced in the quantified accounts of Space Marines equipment (i.e how effective Boltguns are in comparison to todays weapons). I struggle to think of anything that is anywhere near capable of beating them, outside of massively outweighed numbers, or supernatural strength- daemons, nids, etc.
I tend to see Spartans as vastly superior custodes, in that they operate individually (after reach was glassed), hinging upon their superior intellect, physicality, and tactics.
I by no means discredit the speed, strength, and power of a astartes, it is just that the spartans showed feats that trumped anything I have seen of any space marine (barring special characters).
Some people like to discredit the game as canon, yet it is canon when it comes to the halo mythos. Flipping 72 ton scorpion tanks (in game), to survivng dropping from the atmosphere without armor (books), to being able to rip elites apart by the dozens (books).
I think what needs to be established here, really is the power, strength, and speed of an elite, to demonstrate just how powerful spartans are by comparison (fall of reach).
Speed wise spartans were able to outrun covenant hover bikes, and out maneuver incoming missiles, rockets, and other ordinance.
I just take it as I see it, and spartans trump astartes in individual power, however in a war a legion/chapter would win due to superior numbers and space craft.
A better comparison would have been the Human Federation of Old (the one which fought the forerunners) Versus the Imperium of man.