If you're comparing this with the PX100-II, this model has bigger earcups with more padding -- the PX 100-II headphones feature simple foam pads like you'd find on a generic model. Also the PX 100-II has an open-back design that tends to leak more sound than this model, which features a closed supra-aural design.
It's also worth noting that like the PX100 II, this model's 4-foot wire terminates in a vertical plug -- I prefer an L-shaped plug since it usually lasts longer because you're not constantly pulling directly on the cord connection point. Unlike the Bose OE2s, the cord of the HD 238 isn't detachable from the earcup, and it's arguably a tad thin.
While the HD 238 headphones don't isolate noise as well as over-the-ear models, they do well to muffle outside noises -- this is called passive noise cancellation, in contrast to the active noise cancellation of some headphones that requires an internal battery to work.
In reviewing the Bose OE2, I noted that they sound more natural and don't suffer from "Bose bloat," a term coined for its obtrusive bass boost. These Sennheisers exhibit similarly detailed sound and ample but not "bloated" bass. These are a touch on the warm side and will appeal to listeners who prefer more-laid-back, less-aggressive headphones with edgier treble.
Keep in mind that you shouldn't expect the same kind of sound from these Sennheisers as you would with higher-end models that cost well over $200. The point here is that the while the 238s don't offer great sound, they do deliver very solid performance for their price.
The Sennheiser HD 238 headphones actually carry a list price of $139.95, $10 less than the Bose headphones' list price. Bose headphones are very infrequently discounted, however, and a quick CNET Shopping search finds these Sennheisers for less than $75.
The problem is, that deal is unlikely to last. These headphones have been discontinued, presumably replaced by the Sennheiser HD 239s (list price $120). (Sennheiser also makes the