Though probably not as rare as the 1.5MT of the Toyo, Chris, the 0MT used in Cowells spindles isn't exactly common, particularly as it's a shortened version. A true 0MT is 2" long, with diameters of 0.250" at the thin end and about 0.356 at the other. The Cowells version is about 1.125" long, and comprises only the thinner end of a standard 0MT, so if you stick a standard 0MT into a Cowells spindle (or tailstock), about 7/8" of it will protrude.
That said, I suppose the spindle taper wouldn't be relevant if you concoct a screw-on collet chuck for working on your clocks. I'm not sure how things are attached to the Toyo spindle; your picture (which bears a remarkable similarity to the one in a well-known archive) seems to hint that bolts pass through chucks etc from the front into tapped holes in the spindle flange. If so, it wouldn't be too hard to make up a collet chuck for the Toyo, either.
One thing both have in common is the lack of any half-nuts, so the carriage must be moved either by handcranking or under power feed (assuming the Toyo you are looking at comes with fine feed gears). On my Perris (which became the first Cowells 90ME), going any distance with an M10x1mm feedscrew was so exasperating that I made a half-nut for it. Of course, horology probably doesn't entail using the full length of the bed very often.
As the owner of a Perris, I think I'd choose the Toyo. It looks better engineered. But £350 for a Cowells is good value nowadays, and the Toyo looks a nice machine for £300, if someone hasn't snapped it up already.
Andy
Edited to get rid of typos