Yes, there is some overlap in this regard. At it's worst, a serf is little better off than a slave. This wasn't standard practice however, and the life of a serf wasn't actually horrible for the most part. It was tolerable, and it was in the interest of the lord to be such, peasant uprisings are an unnecessary expense and don't bring you any good ransoms.
McHrozni
As usual, when talking about a thousand years and a VERY large surface, things could vary a lot. People shouldn't confuse high medieval England with, say, 18'th century Russia, for example. The two are very different extremes.
In Russia, Tsar Peter The Great "emancipated" all slaves into serfs, but basically reduced the little rights all serfs had to slave levels. So basically in everything but name only, he enslaved all serfs.
This resulted in such extremes as that could just buy large numbers of slaves and put them on literal death marches to St Petersburg, IN CHAINS, to build him a grand palace. In fact, he conscripted 40,000 serfs PER YEAR, which works out to about one for every nine to sixteen serf households in Russia at the time. Why so many per year? High mortality rate. No, really, he worked about as many to death per year, plus the losses on those death marches, etc. You know, fun times.
Oh, and they had to provide their own food for the road and tools for their work.
But generally, things varied a lot across the vast expanse we're talking about. While for example in England they preferred to work things out in money pretty early, in Russia they rather preferred to be paid in work. (Yes, some paid the
obrok in money instead, but generally they were only about a quarter of the population in the agricultural areas.) It was also entirely unregulated (before 1797) how much the lord has to pay or get paid, and you weren't exactly free to go look for other lords to work for. The
barschina, i.e., corveé of Russian serfs increased steadily over centuries, to the point where foreign travellers to Russia in the 18'th century were consistently
appalled by the onerous work obligations imposed on the peasants ON TOP of having to work the alloted land (because they had to subsist on the crop from that allotted land.)
In an 1797
ukaz (decree) of the tsar, a corvee of 3 days a week was decreed to be enough. Remember, these are dusk to dawn days, so we're talking about roughly the equivalent of a modern 40 hour week... ON TOP of having to work the allotted land. And I have a feeling that the decree wasn't because the lords were charging too little, if you get my drift.
So, yeah, serfdom could be basically every bit as bad as slavery, depending on the time and place. In fact, it could be worse than slavery in, say, Ancient Egypt.