The Cardinal is also a good option. It has spectacular scenery and goes via DC, so there is more flexibility for a stopover than with the Capitol. Taking the latter, the only logical stopover is D.C. (unless you want to change trains twice). With the Cardinal, you can take it non-stop or you have the option of stopping in any city between Alexandria and NY, as those offer transfers to the Regional.
It also bears noting that the OP can change their trip whenever they want, at no charge (other than any price difference). So if you change your mind about the preferred route, you can always change your ticket.