Besides having a plastic trigger group, rough chambers, and a rust-prone finish, the 870 express also has plastic magazine cap and spring retainer instead of the ball detent on the barrel lug and steel spring cup retainer.
This setup uses a dimpled magazine tube to hold the retainer in, which prevents a magazine extension from functioning unless the dimples are removed. In addition, the lack of a ball detent means that if you add an extension, you will have to either have a ball detent installed or use blue Loctite on the threads to hold it on.
Even if you have no intention of fitting a magazine extension, the Express retainer setup is still cheap and junky, as well as being "non standard" compared to other 870 models.
This does not apply to the Express models with extended magazines. These guns use the conventional ball detent on the barrel lug.
If you must buy new 870, get either the Wingmaster or Police models so as not to encourage Remington to continue to make shoddy products. If cost is an issue, there are lots of reasonably priced used 870s kicking around, which can be readily modified to your requirements.