There is an intermediate form of MTM that is worth mentioning.
The most commonly known form of MTM--the mainstay of department stores and local haberdashers--is, for want of a better term, taking measurements and then sending it on to an offsite manufactury. The garment is then returned to the shop that takes the measurements, and if (1) the measurements were taken expertly and (2) the person or people who make the garment understand and agree with the communication of the measurements, one might end up with a garment that fits better than OTR. It seems somewhat common, however, that such MTM garments come back, and then are often altered again...ironically, almost as if they were OTR garments in the first place.
There is a subset of MTM, however, that still exists. In this subset, the person who takes your measurements (or someone in the shop) cuts the garment on premises. The cut pieces are then sent out to a manufactury, where it is assembled and returned. I believe Paul Winston (a.k.a. Chipp 2) offers something like this as his MTM, and I think Kilgour had an operation like this, which they called something like "Entry level bespoke," where the cloth was cut in London and then sewn up in China. Or the person who measures you might manipulate the block pattern
directly locally, and rather than sending on simple measurements, send on an altered pattern to the manufactury for cutting and assembly (this last is the type of MTM that my bespoke tailor offers, although I have not used it).
In the more common method, the salesman or measurement taker is not directly involved in manipulating the block pattern. In fact, the person measuring you might be a clerk or a dude who owns the shop, and not a tailor, cutter or stylist. A stranger (or maybe a group of strangers) who has never seen you adjusts the pattern, and does so through (one hopes and prays) through a dry interpretation of numbers on page written up by the person who deals with you directly.
In the less common method, one is getting very close to a bespoke garment. Typically, cost savings are achieved because the quality of manufacturing (for example, the proportion of the garment sewn by hand), is ususally less than in a full-on bespoke item. And it is often the case that the offsite maker might be a large operation making a large volume of RTW in addition to taking in MTM orders, and so, have economies of scale that translate to lower prices.
Some top bespoke makers offer an MTM option, and it is quite possible to benefit from bespoke skill and insight at lower cost, whether the maker cuts your garment, or just modifies the block pattern, since they will subsequently fit the completed garment on you.