Somebody says the 25xx series, having a larger diameter, allows for a more uniform development of film. That's because in a tank the inner coils have a different agitation pattern than outer coils. That's, IIRC, the argument.
I understand the theory but in practice I use only 15xx and cannot detect any problem in uniformity of development.
15xx series also uses a lower volume of chemicals.
If you are beginning now buying your tanks starting with 25xx might be a good idea if you lean toward the perfectionist. Perfectionism sometimes is not about actual results but "peace of mind" that you are adopting the best techniques and not "leaving on the table" the last bit of quality.
If you already have 15xx tanks replacing them with 25xx doesn't make a lot of sense on real practical ground IMO.