Hypothetically, it is possible to have considerably nicer formatting and display in ePub than Mobi (see the screenshots in the thread for this illustrated version
of Three Men in a Boat
in our Patricia Clark Memorial Library if curious).
On a practical level, pretty much no one really bothers with taking advantage of ePub's display capabilities and most readily-available popular fiction books will look almost the same in either format. (Sometimes there will be conversion oddities to Mobi, though, such as intended drop caps making the first line of a paragraph display with a break below the much larger first letter, which tends to show up in some HarperCollins titles, along with tiny squinty font).
The new KF8 format for newer Kindle models is supposed to bridge the display gap, however, comparatively few books are available in it and it seems unlikely that publishers will update their old files with a better version to take advantage of the features.
The ebook pricing differential is generally not for quality/feature advantage, but because sometimes the stores can set their own discount pricing, so you usually have to shop around regardless.
In terms of future-proofing, Amazon does occasionally change the DRM scheme for their e-books and break the tools that people use for removing it so they can perform backups of their paid purchases, on an average of about twice a year or so, so I'd personally consider that to be something to keep in mind, though YMMV.
Hope this helps, and welcome to MobileRead!