Contractor vs. window installer depends on the level of replacement you are searching for. If you are looking to have the windows 100% removed and new full unit windows installed I would recommend a contractor. If you are looking for the more basic window insert replacement a contractor will be your best bet, but a window company should also provide good results. The removal of the old frames is accomplished several different ways depending on the style of window you have. I have good success with collapsing the frames inward towards the center of the opening after removing the sashes. Usually the aluminum will tear at the nail holes and are able to be torn out. Takes some muscle and patience but is usually possible. If that does not work or proves too difficult, a reciprocating saw can cut out the frame of the window leaving the fins buried in the wall. This, too, is somewhat time consuming, but very possible. Either way it takes a fair amount of banging, vibration and abuse (literally) to get them out. If your brick is in good shape it should survive very well, needing only very minor repairs to loose mortar.