After 1066 and shortly thereafter they were more or less the same ...
The varangians had the advantage of far better training, unit cohesion and equipment. The roman heritage of professional military training and strategey still lived in Constantinople, and could work wonders on the gigantic barbaric Franks that signed up for the Guard.
Varangians wouldn't break ranks to chase retreating Normand cavalry, for example. And they lived and trained together all the time, while housecarls lived separately and seldom gathered.
But I also agree on the "let us hire them instead of fighting them" approach, which was highly byzntine.