Isolation Transformation question

I've seen descriptions about making an isolation by using 2 identical transformers back to back, but I've seen transformers that have a primary of 115v and a secondary of 115v. It would seem that this is a better solution (Hammond 169 PS). Am I wrong?

It's certainly simpler to use a single 1:1 transformer, but it's usually more expensive, since the transformer has to be able to handle the load you expect. 50 watt isolation transformers aren't too big or expensive, relatively, but that size will only handle fairly small (maybe 7 tubes) radios. If you expect to service larger (more tubes) radios, or TVs, then you'll need to handle at least a couple hundred watts. That gets expensive, and bulky. The back-to-back transformer idea is bulky too (each still has to handle the power) but sometimes used transformers can be acquired at scrap prices at hamfests or electronic flea markets.

Thanks for the quick answer!