I dunno. I went through this whole debate and decided on archival grade discs with scratch proof coating. The archival discs are rated for 100 years of data integrity, and the scratch proof coatings are VERY VERY strong. You basically need a very sharp knife and serious pressure to scratch them.
Also, usb drives are NOT archival AT ALL! They suffer from something called bit rot, where the data just eventually corrupts and is lost, even when properly stored and never touched. 99.999% of USB keys are also not write-only like a finalized DVD is, which means they can easily erase all the photos on them by accident. While DVDs may be nearing extinction, the form factor is still alive and well in the form of blurays, with backwards compatibility to dvd. USB may not even exist in 10 years, so it's no better than DVD as far as obsolescence goes... Who knows?
It's not like people would carry around their wedding dvd to share their pics anyhow, they would download them to their computer immediately and probably NEVER use the disc again until they accidentally erase them or something. So really, is it that much of a problem that its bigger and clunkier than a usb drive? Or that many laptops no longer have DVD drives? People load up the photos and copy over the net, post on facebook, email them, and share them on their phones and tablets. They just do not need this disc after the first download, so speed really isnt much of an issue either.
I just would rather give them something TRULY archival that cannot accidentally be erased, formatted, or corrupted by clicking the wrong button.
Our clients are totally wowed by our DVD delivery, so making an impression has nothing to do with being DVD or USB or not, it's all in the packaging.
I learned my first lesson regarding non-archival media years ago. Regular CD-R discs (the non-archival type) only last for about 5 years. After the 5 year mark, I was getting a lot of phone calls from old clients who could no longer read their CDs. What a super PITA to dig out all those old files and re-burn them all! I anticipate those using USB keys may go through a similar headache. Since them I've only used archival grade discs.