That's a really good question.
Hypoxia is inadequate oxygen supply at the cellular (or, better, mitochondrial) level. By contrast, ischaemia is a lack of blood supply to a cell/tissue/organ/etc. Let's assume, for simplicity, that patient A's cells are exposed to total hypoxia (i.e. anoxia - absolutely no oxygen), whereas patient B's cells are exposed to total ischaemia. Which is worse?
As it turns out, patient B is in considerably more trouble, all other things being equal.
Patient A suffers only hypoxia. Hypoxia is fundamentally detrimental to our cells because it forces them to metabolise anaerobically (in the absence of oxygen). Metabolising in this manner produces energy (ATP) much less efficiently than can be formed in the presence of oxygen, and energy is thus often not produced quickly enough to meet a cell's needs.
So patient A isn't sitting pretty either, but why is patient B even worse? Well, if you cut off blood flow to a cell, you obviously induce hypoxia there too, since the blood stream normally carries fresh oxygen to the tissues. However, you also cut off all nutrient supplies to the cell. Thus, unless it stores nutrients (e.g glycogen), a cell can't even undergo anaerobic metabolism for very long. Once the local nutrients like glucose are used up, the cell can't do anything more to produce ATP.
So, all other things being equal, ischaemia is worse than hypoxia for a cell, measure for measure.