You can usually get the M373's a lot cheaper if you shop around,
FFX do them for £55 at the moment and often sell the individual larger chisels as clearance though their Ebay store. I have a set of the M373's and the MS500's in my general toolboxes and whilst they are perfectly good chisels, not sure I would recommend them for joinery.
At the risk of muddying the options further, another contender to consider is Japanese chisels. Workshop Heaven do a good range of Fujikawa sets at several price points. Easier to get going and sharpen (albeit with it's own compromises) and a nice addition to any tool box.