I've only been shooting with Canon gear for 15 years (although I have a number of the vintage FL/FD lenses and cameras in my collection)...
Never heard of 250T or 500T close-up lenses. Presumably those are just earlier versions of the current 250D and 500D diopters available today. I have the 500D (the weaker of the two, designed for 135mm and longer lenses) in 77mm size for use on my 70-200/2.8 and 300/4 lenses. Canon also offers a cheaper single-element "500", I believe.
"500" and "250" apparently refer to the change to infinity focus. In other words, with the close-up lens attached a lens can no longer focus to infinity, but only to 500mm (or cm?). Thus the 250 is the stronger of the two.
The 250D is supposed to be designed for use on shorter telephotos, under 135mm.
The 250D comes in one or two diameters smaller than the 500D. I know the 250D can be bought in 52mm, while the 500D can't. The opposite is true in larger sizes. 250D is not made in 72mm and 77mm diameters, for example.
AFAIK, neither 250D nor 500D is available in either 62mm or 67mm diameter, which are becoming increasingly common filter thread sizes on Canon lenses. For example, the EF 100/2.8 USM macro lens uses 58mm size (so can be fitted with either 250D or 500D, which are both available in that size. However, the 100/2.8L IS USM macro lens uses 67mm filters, so there is no 250D or 500D that can be used with it. Neither is available in 82mm or larger diameter, either. I suppose you could use step rings, if you had to.
Personally I prefer to use macro extension tubes instead, whenever possible. I just think image quality tends to be better than when add'l optics are stuck in front of a lens.
Nikon also has offered some excellent close-up lenses over the years.... I'm not familiar with their designations, but it seems a wider variety of strengths and diameter than Canon offers.
There's a good article about the Canon close-up lenses on their European website... doesn't talk about the "T" specifically, but presumably they would be similar to the "D" type that are discussed in detail. Link:
http://cpn.canon-europe.com/content/education/infobank/lenses/close-up_lenses.do